HK1119192B - Separation of contaminants from polyester materials - Google Patents

Description

Separation of polyester material from impurities

Background

Polyesters are polymeric materials prepared by esterification of polybasic organic acids and polybasic acids. The polyester most commonly prepared and used is perhaps polyethylene terephthalate (PET), which can be made by reacting terephthalic acid with ethylene glycol.

Polyesters are currently being used in increasing amounts in various applications. For example, polyesters are commonly used in the manufacture of containers for a wide variety of beverages and food products, photographic films, X-ray films, magnetic tapes, electrical insulation, surgical instruments such as synthetic arteries, fabrics, and other textiles.

Since polyesters can be remelted and reshaped, efforts are being made to recycle as much of the polyester as possible after it has been used. However, before the polyester is recycled, it is necessary to separate impurities (i.e., substances found mixed with or associated with the polyester) from the used polyester. For example, the impurities may be loose and mixed with the polyester material, may be associated with a surface of the polyester material, such as a label affixed to the surface of the material, or may be internal to the polyester material, as in the case of embedded or entrapped materials.

There is a need in the art for improved methods of removing impurities from polyester materials, and in particular from used polyester materials.

Summary of The Invention

In general, the disclosed invention relates to a process for separating polyester from impurities. In particular, the disclosed process enables the separation of the polyester from the following impurities: impurities embedded or entrapped in the polyester, impurities associated with the surface of the polyester and/or impurities only mixed with the polyester.

The process may be described as a multi-stage process comprising a preparation stage and a reaction stage. If desired, the preparatory phase may include operations for chopping the polyester and forming a mixture including the polyester and impurities. For example, in one embodiment, the polyester may be chopped into flakes having a size of less than about 15 mm.

The preparatory phase may include various operations for physically removing a portion of the contaminants from the polyester mixture, such as, for example, one or more elutriation (elutriation) processes in which various loose contaminants, including, for example, metal and paper contaminants, may be removed from the mixture, or dry cleaning operations in which contaminants may be removed from a substantially dry mixture by fluidizing the dry mixture and rotating the dry mixture about an axis of a cleaning chamber to impart a rotational movement to the mixture about the axisPart of the mixture collides with the screen aperture walls and a portion of the impurities is allowed to pass through the screen aperture. Another dry separation method that may be included in this preparatory stage is the color sorting (color sorting) method. The pigment sorting method removes various impurities during the preparation stage, including, for example, metals and colored PET, e.g., TiO-containing₂The PET of (1).

In one embodiment, the mixture may be subjected to high quality metal detection and removal operations. According to this particular embodiment, multiple metal detectors may be placed in series or in parallel to form a metal detector group. The mixture is then fed through one or more metal detector groups to remove metal impurities. On each metal detector of the set, the stream can be separated into accepted material (those going to the next step in the process) and rejected material (those containing metals). In one embodiment, the metal may be recovered by this metal removal operation. In those embodiments where two or more individual metal detectors are arranged in a series combination, the sensitivity of the series of metal detectors may increase with the number of series stages. High quality metal detection and removal operations not only increase the amount of metal removed from the product stream, but also, with the addition of the recycle stream, reduce the amount of polyester that can be lost from the stream during this separation process, as compared to previously known processes.

In addition to the separation operations that are typically performed when the mixture is dry, the preparatory stages of the method may include one or more aqueous separation operations. For example, one or more aqueous separation operations such as high intensity washing operations and immersion/floatation operations may be used. In one embodiment, the process may be further improved by recycling the water used in the aqueous separation operation through a recycle tank.

After the preparation phase, the mixture may be processed according to a reaction phase, wherein the reaction phase comprises a high energy mixing operation and a heat setting operation. During the high energy mixing operation, a slurry comprising the polyester/impurities mixture and the alkaline composition can be formed during or optionally prior to feeding to the high energy mixer. For example, the alkaline composition may include sodium hydroxide, calcium hydroxide, magnesium hydroxide, potassium hydroxide, lithium hydroxide, or mixtures thereof. The amount of alkaline composition added to the slurry may include the amount of alkali reacted with less than all of the polyester. For example, in one embodiment, the amount of the alkaline composition that can be combined with the mixture is sufficient to react with less than about 20 weight percent of the polyester.

If desired, the slurry may be formed in a more conventional mixer prior to being fed to the high energy mixer for reaction. For example, the alkaline composition and the dried polyester mixture may first be mixed in a low energy mixer before being fed to the high energy mixer. During the low energy mixing process, the polyester flakes contained in the slurry may be coated with the alkaline composition. Further, in one embodiment, the energy input to the slurry contained in the low energy mixer can be controlled so that the alkaline composition can react with the specific impurities included in the polyester blend. For example, the conditions of the low energy mixer may be maintained to promote the reaction of aluminum impurities with the alkaline composition while the slurry is in the low energy mixer.

The high energy mixer used may be one that imparts sufficient energy from the mixing action itself to the slurry to promote the saponification reaction between a portion of the polyester and the alkaline composition. In particular, any heat added to the high energy mixer during the high energy mixing operation is insufficient to independently provide sufficient energy to promote the saponification reaction.

After this high energy mixing operation, the mixture may be further processed, for example, in those embodiments where food grade polyester materials are desired for the polyester product of the process. In one embodiment, after saponification in the high energy mixer, the slurry may be heated to a temperature not exceeding the melting point of the polyester. In particular, the mixture may be heated in an environment containing less than about 80ppm water content. Optionally, the mixture can be heated in an oxygen deficient environment. However, this is not a requirement of the invention and in other embodiments the mixture may be heated in an oxygen rich environment.

If desired, the mixture may be preheated prior to the heating step, for example to dry the mixture. According to this embodiment, the mixture may be preheated to a temperature of less than about 160 ℃.

The process may also include operations for recovering various by-products produced during the separation and/or reaction operations. For example, in one embodiment, ethylene glycol is produced during the saponification reaction. If desired, the ethylene glycol can be recovered after the saponification reaction. Another by-product that can be produced and recovered in the process, if desired, is terephthalate, which can be produced in the saponification reaction.

The process can be advantageously used to remove many impurities from polyesters that are difficult to separate. For example, in one embodiment wherein the impurities comprise polyvinyl chloride, a portion of the alkaline composition reacts with the polyvinyl chloride, causing the polyvinyl chloride to become dechlorinated during the reaction.

In the past, aluminum has been another impurity that has proven difficult to separate from used polyester. According to one embodiment of the invention, aluminum mixed with the polyester may be reacted with a portion of the alkaline composition and removed from the mixture, for example, as an aluminum salt or as a fragile aluminum residue of the reaction.

Brief Description of Drawings

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the remainder of the specification, which makes reference to the appended figures, in which:

FIG. 1 illustrates a flow diagram of one embodiment of a polyester recycling process according to the present invention; and

FIG. 2 illustrates a flow diagram of one embodiment of a multi-step high quality metal removal system according to the present invention.

Detailed description of the preferred embodiments

Reference will now be made in detail to various embodiments of the invention. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used on another embodiment to yield a still further embodiment. It is therefore intended that the present invention includes such modifications and variations as come within the scope of the appended claims and their equivalents.

The present invention generally relates to a process for separating and recovering used polyester from various contaminant materials. The present invention can also be used to separate used polyester from various impurities including glass, dirt, paper, metal, glue, dyes, etc. Advantageously, the disclosed process includes a plurality of stages, including a preparation stage, wherein a portion of the impurities can be removed from the mixture comprising the impurities and the polyester, and a reaction stage, wherein a portion of the polyester can be saponified thereby allowing the polyester to be separated from other impurities. In particular, impurities physically associated with or within the polyester can be separated during the reaction stage of the process so that they are more easily removed from the mixture during the separation step. In addition, during the reaction stage of the process, certain impurities that are difficult to separate, such as aluminum and/or polyvinyl chloride (PVC), can react to become more easily separable from the polyester. In certain embodiments, the disclosed methods can significantly reduce the overall wastewater production and impurity levels in the wastewater during polyester recycling.

Generally, a polyester is defined as the esterification or reaction product between a polybasic organic acid and a polyhydric alcohol. It is believed that any known polyester or copolyester may be used in the process of the present invention. However, in one particular embodiment, the process of the present invention is directed to a class of polyesters referred to herein as poly (polyol terephthalate) wherein terephthalic acid is used as the polybasic organic acid.

As used herein, a polybasic organic acid refers to any organic acid having two or more carboxyl groups (-COOH). Most polyesters are derived from dibasic acids, also known as dicarboxylic acids. The polyacid may have a linear or cyclic configuration. Examples of linear polybasic acids that can be used to prepare the polyester include aliphatic dicarboxylic acids. In particular, aliphatic dicarboxylic acids having up to ten carbon atoms in their chain may be used. These carboxylic acids include adipic, glutaric, succinic, malonic, oxalic, pimelic, suberic, azelaic, sebacic, maleic and fumaric acids.

On the other hand, the cyclic polybasic organic acid includes a carbocyclic dicarboxylic acid. These acids include phthalic acid, isophthalic acid, and terephthalic acid. In particular, terephthalic acid is used to make polyethylene terephthalate, which is perhaps the most common commercially available polyester.

As described above, the polybasic organic acid may be reacted with a polyol to form a polyester. Polyols are compounds containing at least two hydroxyl groups. Many polyesters are synthesized as diols. Diols are generally prepared from olefins by the net addition of two hydroxyl groups to the double bond in a process known as hydroxylation. Polyols are generally referred to as diols and polyols. Examples of polyols for ester polyesters include ethylene glycol, propylene glycol, butylene glycol, and cyclohexanedimethanol.

For purposes of example, table 1 contains a non-exhaustive list of commercially available polyesters that can be recovered and recycled in accordance with the present invention. For each polyester, the corresponding polybasic organic acid and polyol are provided.

TABLE 1

Polyester	Polybasic organic acids	Diols
			Polyethylene terephthalate	Terephthalic acid (TPA)	Ethylene glycol
Polybutylene terephthalate	Terephthalic acid (TPA)	Butanediol
			PETG copolyester	Terephthalic acid (TPA)	Cyclohexanedimethanol and ethylene glycol
PBTG copolyester	Terephthalic acid (TPA)	Cyclohexanedimethanol and butanediol
			Polycyclohexanedimethylene terephthalate	Terephthalic acid (TPA)	Cyclohexane dimethanol
PEN polyester	Naphthalenedicarboxylic acid	Ethylene glycol

In a particular embodiment of the disclosed invention, the recycled polyester may be polyethylene terephthalate (PET). Accordingly, much of this discussion relates to PET, although this is not to be considered in any way as limiting the invention to the recovery and depuration of PET.

In one embodiment, the process of the present invention may be considered a three stage operation to remove impurities from polyester and may include a preparation stage, a reaction stage, and a finishing stage. Further, each stage of the overall process may include one or more separate operations. In one embodiment, the preparation stage may comprise at least one dry separation operation and at least one aqueous separation operation during which impurities may be removed from the polyester-containing mixture. The preparation phase may be followed by a reaction phase wherein the mixture containing polyester and impurities may be mixed with an alkaline composition. The base may react with a portion of the polyester during saponification and with various possible impurities in the mixture to degrade or chemically convert the impurities to form a material that is more easily separated from the polyester. This stage of the process may also include separating and removing impurities from the mixture, if desired. The reaction stage may also include various heat setting reactions which have many benefits in that the polyester matrix can be further purified and the physical properties of the product material improved. The final finishing stage of the invention may include operations to improve product quality by, for example, washing and sorting of the product, as well as additional separation operations.

The process of the invention can be operated continuously or can be set up as a batch system. Further, any particular operation of the present invention may be run in a continuous or batch system. In fact, any polyester-containing material can be treated in accordance with the present invention. In a preferred embodiment, the polyester material can be recovered from the solid waste stream, thus mitigating many environmental and disposal issues. In a particular embodiment, the present process may involve the recycling and recycling of food and/or beverage containers made from PET. By the process of the present invention, it is possible to separate, recover and reuse polyester from used waste even when it is found to be mixed with certain materials that are difficult to separate, such as polyvinyl chloride or aluminum that adhere to various coatings or are intercalated with various materials such as organic and/or inorganic compounds. Unfortunately, due to the lack of an economical process for separating and recovering polyester, many of the used polyester is currently disposed of in landfills or incinerated after use.

Impurities contained in or adhered to the raw polyester material that can be advantageously removed by the disclosed methods include various barrier materials. The barrier material removable in accordance with the present invention may include a barrier coating which may be, for example, a barrier coating applied to a beverage container to prevent the flow of carbon dioxide and/or oxygen through the substrate. Other barrier materials that may be removed in accordance with the present invention may include certain chemical barrier material additives, such as chemical scavenger materials that are added to the polyester material at the time of initial formation, and/or decomposition products resulting from the reaction of such additives. In one embodiment, the disclosed method may be used to remove barrier materials applied as an interlayer coating on a multilayer bottle.

In addition to the coated barrier material, the present method may also remove other coatings from the polyester material. For example, the method may remove applied labels, including paper and/or polymeric labels, screen printed labels, and the like. The term "screen printed label" generally refers to an ink that has been applied directly to a polyester container, such as a beverage container. For example, many soft drink containers are typically labeled with epoxy-based inks. In the past, many problems have been encountered in attempting to separate these types of coatings and inks from polyester.

A non-limiting, exemplary list of barrier materials (coatings and chemical constituents throughout the polyester matrix) and non-barrier coating materials that may be removed from the polyester in accordance with the present invention may include, for example, saran, nylon, polyvinylidene chloride, acrylics, epoxy-based polymers, acetaldehyde scavengers, ethyl vinyl alcohol (e.g., EVOH film), and the like.

The process of the present invention is also effective in removing entrained organic and/or inorganic compounds that may have been absorbed by the polyester material. Exemplary compounds may include, for example, toluene, gasoline, used motor oil, paint, pesticide residues, and other volatile compounds. These compounds can be absorbed by the polyester only upon contact. Consumers often misuse polyester food and beverage containers after consuming food or beverages. In particular, these containers are sometimes used to hold various organic and/or inorganic compounds and solvents. When attempting to recycle such polyesters, it is necessary to remove substantially all of the absorbed organic and inorganic compounds so that the polyester can be reused as a container for beverages or food.

Advantageously, despite the presence of impurities, the process of the invention may comprise: a first preparation phase comprising the removal of a portion of the impurities via one or more physical separation methods; followed by a reaction stage which comprises, in one embodiment, contacting the polyester containing compound with an alkaline composition, mixing the alkaline composition and the polyester containing mixture together so that the solids of the mixture are sufficiently and uniformly coated with the composition and partial saponification of the polyester occurs, heating the material in a one or more step process to a temperature sufficient to complete the saponification reaction, if desired, among other possible benefits, to maintain and/or enhance the physical properties of the polyester; and purifying the polyester-containing mixture by various possible separation operations including, for example, washing the material with a fluid such as water.

The recovery process according to an embodiment of the present invention is described below with a preferred example in fig. 1. As shown in fig. 1, the disclosed process can be conveniently described as being depicted in three stages, namely a preparation stage, a reaction stage, and a finishing stage. It should be understood, however, that this particular depiction is for convenience in describing this embodiment and should not be taken as a requirement of the disclosed method.

Preparation phase

If desired, the contaminated polyester material may be shredded or ground into flake form in, for example, a shredding operation prior to separation from impurities in accordance with the present invention. For the purposes of this disclosure, the term polyester flake refers to a polyester material that has been shredded or ground into smaller pieces. The flaking of the material may be performed for the purpose of convenient handling. It should be understood that the methods of the present invention may use materials of different sizes and shapes, and do not require one size or shape. For example, in one embodiment, the polyester may be in a discrete form, such as finely divided or pelletized. Examples of the size of the crushed pieces may include, for example, pieces having a size of about 1 to about 15 mm. In one embodiment, the chip size may be from about 0.125 to about 0.75 inches. The exact shape of the fragments is not important to the present invention.

In a particular embodiment, the polyester may be ground or chopped when the mixture containing the polyester is dry. While not wishing to be bound by any theory, it is believed that chopping the material in the dry state can improve the separation of certain impurities from the polyester matrix. For example, it is believed that when multi-layered bottles are treated according to the disclosed method, dry grinding can facilitate separation of the layers and removal of coating material located between the polyester layers in the multi-layered bottles.

After the polyester substrate is chopped or flaked, the dried mixture containing polyester and impurities can be subjected to one or more operations to remove impurities heavier than the polyester. For example, according to one embodiment, the mixture may be subjected to a specially designed elutriation process to remove heavy impurities, and in particular, metallic impurities. Elutriation is simply a method of separating lighter materials from heavier materials by using an incoming fluid stream (i.e., gas or liquid). In the past, panning has been applied in many processes including recycle processes to remove lighter impurities from polyesters. For example, it is known to elutriate operations throughout a cyclical process to remove lighter label material, such as paper, from, for example, a mixture. Furthermore, an elutriation operation may be used throughout various points of benefit in the process of the present invention to remove fines from the mixture.

According to one embodiment of the invention, after chopping or grinding of any desired material, an elutriation process may be used to remove impurities from the mixture that are heavier than the polyester material to be recovered. More specifically, although in the past it has been known to remove lighter impurities from polyester mixtures via a elutriation process, according to this particular embodiment of the invention, the polyester containing mixture may be a lighter stream separated in the elutriation operation, while heavier impurities, such as metal, stone, dust, etc., may be removed from the mixture in the heavy stream exiting the elutriator. Depending on the particular separation operation, the elutriation fluid flow rate may be higher than that used in previously known processes for separating lighter materials from a polyester-containing stream to separate heavier impurities from the polyester-containing mixture. For example, in one embodiment, a flow rate of a fluid stream (e.g., air) of about 3600 to about 4600 cubic feet per minute (cfm) may be used during the elutriation process, while the flow rate of solids introduced is about 2500 to about 3500 pounds per hour (lb/hr) to separate heavier solids from the polyester-containing mixture. In one embodiment, the flow rate of the introduced solids may be about 3000 lb/hr.

Referring to fig. 1, at some point in this preparation stage, for example after the elutriation process for removing heavy contaminants, the mixture containing polyester and contaminants may be charged to a dry cleaning vessel. In particular, the mixture of materials loaded into the dry cleaning container will contain at least some used polyester and the mixture will be dry. I.e. the mixture is not in a slurry state. However, the dried mixture of materials need not be very dry. For example, the mixture need not be pre-treated to remove all of the moisture from the mixture, and the mixture may be charged to the container at atmospheric humidity levels.

The dry cleaning vessel may include a cleaning chamber into which the dry mixture is loaded. In a preferred embodiment, the chamber may be at least partially surrounded by a mesh having a predetermined mesh size. Preferably, in some embodiments, a majority of the individual polyester chips can be loaded into the dry cleaning vessel larger than the openings of the mesh material in the dry cleaning vessel to facilitate handling of the mixture. Crushing the material prior to loading it into the dry cleaning container may also help ensure that at least some contaminants of a certain size pass through the screen openings, although many contaminants will be crushed within the dry cleaning container, as described below.

In one embodiment, the chamber may comprise a series of paddles spaced along the axial length of the chamber. The material, after being loaded into the container, may be guided to rotate or spin around the axis of the chamber, for example due to the rotating action of the paddle. In particular, the movement of the material may be sufficient to fluidize the dried mixture. For example, the paddles may rotate at a speed greater than about 20 meters per second to fluidize the charged mixture. In one embodiment, the blades may rotate at a speed greater than about 2000 rpm. In one embodiment, the tip speed of the blade may be from about 40 to about 100 meters per second, for example about 50 meters per second. In another embodiment, the tip speed of the blade may be from about 60 to about 80 meters/second.

In addition to fluidizing the mixture, the rotation of the mixture caused by the rotating paddles may also promote dynamic collisions of the material in the mixture with the chamber walls. The collision of the material contained in the mixture with the wall can lead to the breaking up of the impurities in the mixture and, in particular, the breaking up of the impurities into individual pieces smaller than the mesh size of the surrounding wall. In addition, these dynamic collisions may be advantageous to physically separate embedded or otherwise attached impurities from the polyester. After the impact between suitably small impurities (smaller than the mesh size of the wall) and the mesh, these impurities can pass through the surrounding mesh and the polyester can remain in the chamber. The rotation of the paddles may also facilitate the flow of air through the chamber and the movement of material from one end of the chamber to the other.

Surprisingly, the dry cleaning operation can physically separate the associated contaminants (including embedded contaminants and the many fragile contaminants) from the polyester without substantial fragmentation of the polyester. For example, when a coating material, such as a paper label material or vapor barrier coating, and an embedded material, such as embedded glass and dirt, can be separated from the polyester substrate during the dry cleaning operation, the polyester sheet itself can retain substantially the same size and shape as when initially loaded into the container. In addition, although contaminants may separate from the polyester and pass through the surrounding screen openings, the polyester may remain in the dry cleaning chamber. Even in those embodiments where the polyester comprises polyester flakes having a size smaller than the mesh openings, a majority of the small polyester fines can remain suspended within the dry cleaning chamber during the separation operation without being lost with the contaminants.

While not wishing to be bound by any particular theory, it is believed that due to the centrifugal forces acting on the suspension, the materials in the mixture may separate, with denser materials and particularly impurities such as glass, metal, paper, etc., moving to the outside of the bulk of the material, while lighter materials, and particularly small polyester fines, may remain suspended near the center of the chamber. Thus, glass-like and fibrous materials may break and pass through a screen at the outer edge of the bulk, while elastomeric materials such as PET may remain behind. In this way, even PET particles smaller than the holes of the screen can remain in the main rotating body, and very little polyester fines can pass through the screen, and a high yield of polyester can leave the dry cleaning operation.

The dry cleaning operation is particularly effective in removing glass from mixtures comprising glass and polyester. Generally, glass has been considered one of the most difficult materials to separate from used polyester during the polyester recycling process, and if not completely removed, it can be detrimental to the process. Glass that is not removed during recycling can not only cause severe damage to processing equipment during the recycling process, but can also damage the materials formed from recycled polyester. For example, glass that is not removed during the recycling process can embed polyester during subsequent material forming processes (e.g., beverage container forming processes) and can damage the material formed from the polyester, such as creating holes in the polyester product.

According to one embodiment of the method, more than about 97% of the glass in the mixture can be removed from the mixture comprising glass and polyester in the disclosed dry cleaning operation. In one embodiment, more than about 98% of the glass in the mixture may be removed. In another embodiment, more than about 99% of the glass in the mixture may be removed in the dry cleaning operation.

In a preparatory phase, for example before or after the dry cleaning operation, the mixture may be further processed according to one or more additional dry separation operations. For example, in one embodiment, the mixture may be subjected to an elutriation operation to remove light impurities from the mixture. For example, the gas flow through the elutriator may be between about 1500 and about 3000cfm for a solids flow rate of about 3000 lb/hr, and at least a portion of the lighter impurities may be removed from the mixture. Other dry separation operations that may optionally be included at one or more locations in the preparatory phase include any separation operation known in the art, such as a screening operation that uses a vibrating screen to separate impurities larger or smaller than the polyester, depending on the mesh size of the screen and the size of the polyester flakes.

In the preparation phase, the mixture may also be subjected to one or more aqueous separation operations. For example, in the embodiment shown in fig. 1, a strong cleaning operation may follow the dry cleaning operation and the elutriation operation. Advantageously, this aqueous separation operation, such as a vigorous washing operation, which may follow due to previous separation operations, may result in (describe) lower impurity levels in the wash water and therefore require less wastewater treatment processes. In particular, since many foreign materials and impurities can be removed by the above-described drying separation operation, problems encountered in the aqueous separation technique in the past (such as coagulation of the separated coating material, or the necessity of expensive water treatment) will be less problematic in the polyester recovery process of the present invention.

For example, after a dry cleaning operation, with or without any additional separation operations such as elutriation, the chemical oxygen demand of the wash water may be reduced by at least about 15% compared to when the wash water is used directly to wash soiled polyester material. Similarly, prior to this aqueous cleaning, the total dissolved solids content of the cleaning water can be reduced by about 30%, the total suspended solids can be reduced by at least about 50%, and oils and greases can be reduced by about 15% through the use of at least a dry cleaning operation.

The aggressive cleaning step may include adding water to the mixture and subjecting the material contained in the aqueous mixture to high shear to facilitate the removal of certain impurities, such as oligomers, and other organic and inorganic compositions from the surface of the polyester. Generally, high shear washing may include those in which the wash water is more turbulent than standard washing during operation, but the turbulence is sufficiently low to prevent excessive flake damage and/or fines formation. For example, high shear cleaning in accordance with the present invention may include those wherein the cleaning rotor is rotated at a tip speed of from about 500 to about 1000 feet per second (fps). Such high shear cleaners are commercially available in the art from manufacturers such as Reg-Mac, Sorema or B & B.

According to another aqueous separation operation suitable for use in the disclosed method, polyester-containing materials may be immersed in water in order to separate heavier materials, particularly polyester, from less dense or lighter materials. More specifically, polyesters are known to sink in water while other polymers, such as polyolefins, and paper products, are buoyant. Thus, lighter materials can be easily separated from heavier materials when contacted with an appropriate amount of water. Subjecting the material to a sink/float separation step and removing some of the impurities from the mixture not only reduces the amount of material to be treated in the mixture, but also helps to clean the material before further treatment.

When the mixture containing impurities and polyester is sent to a sink/float tank, it is advantageous in certain embodiments to send the mixture below the liquid surface. This eliminates the effect of the surface tension of the water in the tank on the material contained in the mixture and facilitates the sinking of the denser material into the tank.

In order to further improve the disclosed method and, in particular, to reduce the amount of water used in the method, the preparatory phase of the method may also comprise a recirculation tank for recirculating the water used in, for example, intensive washers and/or sink/float tanks. Thus, any water used in the aqueous operation of the preparatory stages, as well as any water removed from the material in any drying operation of any stage of the process, may be recycled through the recycle water tank. Furthermore, if desired, the recirculation tank may include an agitator to maintain the suspension of any polyester fines entering the recirculation tank and thus facilitate reintroduction of the polyester fines back into the main stream of the trash removal process.

Other aqueous separation operations that may optionally be used in the preparation stages of the process include, for example, the use of one or more hydrocyclone separators as is well known in the art. For example, a single hydrocyclone, two hydrocyclones in series, or multiple hydrocyclones can be used to separate the polyester containing mixture from the impurities.

After aqueous preparation operations (e.g. intensive washing and sink/float separation steps), the mixture containing polyester and remaining impurities may be dried and optionally subjected to additional drying separation operations before being mixed with alkaline material during the reaction stage of the process. This drying operation may occur, for example, at a temperature of no greater than about 160 ℃. For example, in one embodiment, drying may occur at about 130 to about 160 ℃, and may generally be performed according to any method known in the art.

For example, according to the embodiment illustrated in fig. 1, prior to the reaction stage, the mixture may be dried and then subjected to a pigment sorting operation. It has been found that it is advantageous in certain embodiments of the present invention to use a pigment sorting operation prior to the reaction stage of the process, since this operation can essentially act as a further cleaning step in the process to remove various impurities from the mixture and thereby "clean" the mixture prior to the reaction stage. Impurities that may be removed from the mixture prior to the reaction stage via a pigment sorting operation may include, for example, metals and colored high molecular components such as polyester materials colored with titanium dioxide.

Polyester materials which cannot be effectively recycled for coloration in general, in particular with dioxideTitanium pigmented white polyester materials because they are considered as impurities of the desired clear product stream. Therefore, it is advantageous to remove such materials from the recycle stream. In the past, however, this has proven difficult because transparent polyester feed materials can crystallize and appear white in the recycling process itself and thus are difficult to blend with white impurities such as polyester/TiO₂And (5) separating the materials. According to the presently disclosed method, such materials can be separated from the stream prior to the reaction stage and thus prior to crystallization of the possibly recyclable polyester.

In general, any colorant sorting technique known in the art may be used in the disclosed invention. For example, visual inspection or automated optical sorting and separation techniques may be used. Examples of commercially available optical toner sorting equipment may include those produced by manufacturers such as SRC, Satake and MSS.

Other separation operations useful in the present invention (e.g., during a preparatory stage of the process in order to remove as much of the impurities as possible prior to the reaction stage) may include, for example, the use of aqueous separation processes such as additional elutriation processes, additional washing operations, additional sieving processes, and/or specially designed operations to recover metals from streams.

Metal removal methods suitable for use in the present invention may include the use of magnetic separators, such as magnetic drums, waterfall type magnets, eddy current machines, or any other suitable magnetic metal detector and separator, such as those available from Bunting Magnetics co, of Newton, Kansas or S & S Recycling GmbH, germany. Such devices may employ any type of magnet (e.g., permanent magnet, rare earth magnet, electromagnet) in any suitable design to remove at least a portion of the metal impurities contained in the mixture.

Reaction stage

During the reaction stage of the disclosed process, a portion of the polyester in the mixture may be saponified by the reaction of the polyester with an alkaline compound. More specifically, the reaction stage of the process may include one or more mixers, at least one of which may be a high energy mixer. In addition, the reaction stage may optionally include additional reaction operations, such as high temperature heat setting operations.

For example, and in accordance with the embodiment shown in FIG. 1, the reaction stage of the process may include a first low energy mixing operation wherein the mixture comprising polyester and impurities may be mixed with an alkaline composition prior to being fed to the high energy mixing step operation wherein a portion of the polyester may be saponified. After the high energy mixing operation, the mixture can be dried, if desired, for example using a drying oven and fed to a heat setting operation, wherein, among other benefits, the saponification reaction can be completed. In addition, the second material may further react with the alkaline composition and may maintain and/or enhance the physical properties of the polyester during the heat setting operation.

The reaction stage of the present process may optionally include one or more methods for recovering reaction by-products. For example, after saponification of PET, the reacted polyester material may be converted to metallic terephthalate and ethylene glycol. The thus-produced terephthalate of the metal can be dissolved in water and the water can be acidified, if desired, resulting in precipitation of terephthalic acid. If desired, the terephthalic acid can be filtered and recovered as a by-product of the disclosed process. Similarly, the polyol formed during the reaction may either remain as a liquid in the mixture for subsequent removal or may be directly vaporized if the reaction occurs under conditions that facilitate vaporization of the polyol. If desired, the polyol can then be recovered, for example using a condenser.

In a preferred embodiment, the alkaline compound selected for mixing with the material may be sodium hydroxide (commonly referred to as caustic soda). However, other metal hydroxides or bases may optionally be used in addition to or in place of sodium hydroxide. For example, suitable compounds may include calcium hydroxide, magnesium hydroxide, potassium hydroxide, lithium hydroxide, or mixtures thereof. When used in solution, the metal hydroxide may be mixed with water prior to mixing with the polyester-containing material. For example, in one embodiment, the metal hydroxide and water may be mixed in a ratio of about 1: 1.

Generally, the reaction stage of the present invention includes a high energy mixing step during which the mixture containing impurities and polyester can be mixed with a selected amount of an alkaline solution to form a slurry. The amount of alkaline composition added to the polyester-containing material generally depends on the impurities present in the material and the type and amount of impurities. Generally, the alkaline composition should only be added in an amount sufficient to separate the polyester from impurities in order to minimize saponification of the polyester. In most applications, the alkaline composition may be added to the material in a stoichiometric amount sufficient to react with up to about 50% of the polyester. Preferably, the alkaline composition is added in an amount sufficient to react with less than 10% of the polyester and most preferably with about 3% of the polyester.

In the embodiment shown in fig. 1, the slurry may be formed in the first low energy mixer, but this is not a requirement of the invention. In other embodiments, the slurry may be formed directly in the high energy mixer, and the low energy mixer may not be included. Optionally, a surfactant or wetting agent may be added to the mixture and alkaline composition when forming the slurry. The addition of a surfactant can facilitate the mixing of the alkaline composition with the material, reducing the amount of the alkaline composition that needs to be added. The surfactant should be stable to alkali and may be nonionic or anionic in nature. An example of a suitable surfactant is the nonionic surfactant ETHAL TDA-3 sold by Ethox Inc. of Greenville, South Carolina.

In those embodiments where one or more low energy mixers are used to form the slurry, certain low energy reactions may also occur in the mixers. For example, when a low energy mixing operation is used, the mixer may optionally be used at operating parameters (e.g., by adding a small amount of thermal energy by heating the mixer) to promote reaction of the alkaline composition with certain impurities found in the mixture, and in one embodiment, with aluminum. However, in those embodiments where additional energy is added to the mixture in the low energy mixer to promote the reaction between the impurities and the alkaline composition, the total amount of energy added to the mixture at this point should not be sufficient to promote any saponification reaction between the polyester and the alkaline composition. For example, in one embodiment, the low energy mixer can be operated at an internal temperature of about 90 to about 110 ℃ to promote a reaction between any aluminum contained in the mixture and the alkaline composition, without promoting a saponification reaction between the polyester contained in the mixture and the alkaline composition.

It has been found that under low energy conditions, such as those that can be promoted in a low energy mixer, the alkaline composition can react with the aluminum in the mixture to produce an alkaline aluminum salt (alkali aluminum salt) that is soluble in the water in the slurry, for example. Thus, in certain embodiments of the invention, a low energy mixer may be used to form a slurry prior to any saponification of the polyester contained in the mixture and to effectively coat the solids in the mixture with the alkaline composition and to promote the reaction of the alkaline composition with various impurities contained in the mixture.

Optionally, a washing or scrubbing operation may be included after the low energy mixer, for example to remove any reaction products formed in the low energy mixer. For example, in those embodiments where aluminum reacts with the basic composition in a low energy mixer to form a basic aluminum salt, the mixture may be washed to remove at least a portion of the salt prior to additional operations in the reaction stage of the present process.

After any optional low energy mixing operation, the materials may be fed into one or more high energy mixers. As previously mentioned, in those embodiments that do not use a low energy mixer to form the slurry, the slurry may be formed in the high energy mixer itself. Also, the alkaline composition may be added to the mixture in the high energy mixer not only in those embodiments where the slurry is formed in a high energy mixer, but also in those embodiments where the alkaline-containing slurry is washed to remove impurities prior to flowing into the high energy mixer or whether or not washing occurs to replace the alkaline composition that reacts with impurities in the low energy mixer.

When desired, the high energy mixer used can not only coat the polyester material with the alkaline composition substantially completely and uniformly, but can impart sufficient energy to cause a portion of the polyester to saponify (or in other words, hydrolyze) without the necessity of adding a large amount (or in certain embodiments, any) of heat to the mixer. For example, a mixer (such as those described in U.S. Pat. No.4,320,979 to Lucke and U.S. Pat. No.4,189,242 to Luke, which are incorporated herein by reference in their entirety) may be used in a high energy mixing operation to facilitate saponification of at least a portion of the polyester with an alkaline solution.

In one embodiment of the invention, the high energy mixer may be operated at a froude number greater than about 4.2, specifically greater than 6.6, and more specifically greater than about 9.5. In particular, at the above rates, the mixer of the present invention not only mixes the slurry but also imparts sufficient energy to the slurry to cause the alkaline composition to react with the polyester. In one embodiment, the high energy mixing may be continued until substantially all of the alkaline composition is consumed. For example, the high-energy mixer can be operated as follows: the residual (unreacted) metal hydroxide leaving the mixer may be less than about 1 wt% of the weight of the slurry. In particular, the residual metal hydroxide leaving the mixer may be less than about 0.5 wt%. More particularly, the residual metal hydroxide may be less than about 0.1 wt%, such as less than about 0.05 wt%, of the weight of the slurry.

During saponification, various coatings that may adhere to the polyester and/or other contaminants that may be entrained on the surface of the polyester can be released from the polyester. The energy provided by the action of the mixer may also promote the reaction between the alkaline solution and other impurities that may be found in the slurry, such as polyvinyl chloride or aluminum. After these types of materials are reacted with the alkaline composition, the impurities may be converted to another form, which is more easily separated from the polyester substrate.

Furthermore, it is also believed that the reaction product salt may form a coating on the polyester material exiting the mixer due to the thorough mixing in the high energy mixer and the application of the substantially uniform base coating to the polyester material. For example, if the outer surface of a PET sheet is saponified with a sodium hydroxide composition in a high energy mixer, it is believed that the reaction product, disodium terephthalate, coats the remaining PET. Further, it is believed that such a coating formed around the polyester sheet can serve to protect the polyester during subsequent processing operations. For example, the salt coating may protect the polyester from oxidation due to high temperature conditions in later-encountered heat-setting operations. Among other benefits, this can provide polyester products that have less discoloration than has been obtained in the past.

In certain embodiments, after the reaction in the one or more high energy mixers, the mixture may be dried and proceed to the finishing stage or optionally directly from the high energy mixer to the finishing stage. In particular, in those embodiments where recycled polyester product from the process is not intended for use as a food grade product, it may not be necessary to use other reaction operations, particularly the heat setting operation discussed below. However, in those embodiments where a food grade product is desired, the process will include at least one heat setting operation.

After the reaction in the high energy mixer, the slurry may then be dried and, in certain embodiments, fed into a heat setting operation. For example, the mixture may be first dried by heating the mixture to about 150 to about 160 ℃, and after drying, heating is continued to a heat-set temperature that may promote other reactions of the material. The actual temperature to which the mixture is heated during the heat setting operation may depend on a variety of factors. Generally, the mixture should be heated to as high a temperature as possible without melting the polyester. For example, PET typically has a melting point of 250 to about 270 ℃. Accordingly, when the material contains a significant amount of PET, the mixture should be heated to less than about 270 ℃ during the heat setting operation. In most instances, the temperature will be in the range of from about 100 to about 270 ℃.

In general, canThe heat-setting operation is carried out in an at least substantially water-free environment, for example, in a dry air environment. However, although a dry air environment is preferred in certain embodiments, inert atmospheres such as nitrogen, argon, carbon dioxide, and the like, for example in the form of a nitrogen blanket, may also be effectively employed due to, for example, cost considerations. If desired, the mixture may also be heated under reduced pressure with a lower oxygen content. The term "at least substantially free" means that the amount of water present in the environment during heating is below that which would cause degradation of the polyester. The amount is generally not more than 80ppm (-40)Lower dew point), preferably no more than about 10ppm, more preferably no more than about 5ppm (-80)Lower dew point). The amount of water is not theoretically minimal and can be as low as 1ppm or even lower than ambient.

Also, in one embodiment, it may be preferable to heat the mixture in an oxygen deficient environment. As used herein, "anoxic" refers to an environment in which oxygen is present at less than about 19% by volume. In combination with a dry atmosphere, maintaining a lower oxygen content during the heat-setting stage also prevents substantial degradation or discoloration of the polymer, and also prevents uncontrolled combustion. If desired, the mixture may also be heated under reduced pressure with a lower oxygen content. In addition, during heating, the slurry is typically heated indirectly so that it does not come into contact with the exposed flame.

However, the present invention does not require an anoxic environment, and in other embodiments, it may be preferable to heat the mixture in an oxygen-rich environment (i.e., an environment having an oxygen content of greater than about 19% by volume).

The equipment and apparatus used during the heat setting operation of the present invention may vary. For example, in one embodiment, the heat setting operation may be performed in a rotary kiln. The rotary kiln may be heated by electrical elements or by heating oil or fossil fuel burners. One example of an indirectly heated kiln suitable for use in the process of the present invention is a rotary calciner sold by Renneburg division of Heyl & Patterson, Inc. In other embodiments, however, a multi-tray thermal processor or furnace may optionally be employed. Of course, many other similar devices are available such as infrared heat processors, microwave heaters, and the like, may optionally be used in the methods of the present invention.

In one embodiment, the kiln may first be heated to a lower temperature and held for a desired period of time to dry the material from the high energy mixer, and then the temperature may be raised to a higher level. Alternatively, the slurry leaving the high energy mixer may be first heated in a dryer (e.g., a ConAir dryer) before it is conveyed to the kiln for the heat setting step at a higher temperature. In another embodiment, a kiln may be used to raise the temperature of the mixture to the desired level relatively quickly, and the hot materials may then be conveyed to a larger volume dryer or any other suitable system for bringing them to a solid state, where they are maintained at the desired temperature for the desired time.

The heat setting operation of the disclosed method can provide a number of beneficial effects. For example, during the heat setting operation, by-products such as ethylene glycol, which is formed either in the high energy mixing in a high energy mixer or during the saponification reaction in the heat setting operation, may be vaporized from the stream. The vaporized material can then be collected, for example in a condenser, if desired, and sent to a suitable treatment operation, such as a water treatment operation, for recovery. In addition, any residual unreacted base entrained from the high energy mixing operation may react with the polyester or other reactive impurities found in the mixture during the heat setting operation.

During heat setting, residual entrained organic and/or inorganic compounds that may be absorbed into the polyester may be removed from the product polymer. In particular, in this heat-setting step, any residual volatile organic and inorganic compounds can be not only substantially removed from the polyester, but, depending on the nature of the compounds, can be completely removed from the stream via suction. By ensuring that substantially all any entrained organic and inorganic compounds are removed, "food grade" polyesters that can be used in an unlimited manner can be recovered. In addition, heating the mixture can degrade the fugitive dried impurities into a more readily separable form to facilitate the final separation of the polyester product from the impurities.

The heat-setting operation may also improve the physical properties of the polyester in the mixture. In particular, it is believed that the heat setting operation increases the clarity and intrinsic viscosity of the product polyester material. For this purpose, the heating step under a dry atmosphere may be carried out for a period of time sufficient to enhance the intrinsic viscosity of the polyester. For example, according to one embodiment of the presently disclosed method, the intrinsic viscosity of the polyester in the feedstock may be increased from about 0.76dL/g to about 0.82 dL/g. For example, according to one embodiment of the presently disclosed invention, the intrinsic viscosity of the feedstock material may be increased by about 5 to about 10%. The minimum time depends on, for example, the water content in the environment and can be as low as 5-10 minutes.

Once physically separated from the polyester, impurities, such as the now-shed coating material and/or entrained material, may further degrade as the material undergoes subsequent handling. For example, the solvent and liquid contained in the dope or removed from the polyester during the heat setting operation can be evaporated and optionally removed from the kiln and recollected in a condenser similar to the process used to recollect the ethylene glycol product from the saponification reaction described above. What may remain in the mixture may be some impurities of relatively smaller size. When the mixture is subsequently subjected to other separation operations, such as in the finishing stage of the process, for example using a screen of suitable size to allow passage of the impurities while preventing passage of the polyester, the remaining insoluble impurities can be separated from the larger polyester chips.

During the reaction stage of the process, in addition to saponifying a portion of the polyester, certain impurities that are often found in used polyesters that are difficult to separate can be converted to a form that is more easily separated from the mixture. In particular, impurities such as polyvinyl chloride, polylactic acid (PLA) and aluminum can be converted to a form that is more easily separated from the polyester during the reaction stage of the process.

When polyvinyl chloride and polylactic acid are present in the material, the material may be converted into a form that is more easily separated from the mixture. For example, according to one embodiment, at least a portion of the PVC may be converted to a darker form and thus may be separated from the mixture by colorant sorting techniques. In certain embodiments, the PVC impurities may be converted to a form that floats in water. In other embodiments, the PVC may be reacted to form forms that exhibit increased heat resistance. Moreover, a combination of these features can be demonstrated by reacted PVC. In general, the specific reaction of the PVC with the alkaline composition depends on the specific nature of the PVC impurities contained in the mixture. However, regardless of the initial nature of the PVC impurity, it is believed that upon mixing the mixture with the alkaline composition and adding the appropriate amount of energy, the PVC can react to form a form that is more easily separated from the polyester contained in the mixture. Thus, when polyvinyl chloride is present in the material, it is preferred to add to the slurry sufficient alkaline composition to react with the polyvinyl chloride (or in other words, to convert the polyvinyl chloride to a form that can be separated from the polyester).

However, if the PVC fails to react with the alkaline composition in the high energy mixer and/or during heat setting, the PVC may optionally be removed from the mixture by heating the mixture to a temperature above the melting temperature of the PVC but below the melting temperature of the polyester contained in the mixture, whereby the PVC may be melted and then removed from the mixture either in the reaction stage or optionally in the finishing stage of the process by using a screen or other suitable separation technique.

PLA can be removed from the mixture in a similar manner to that described above for PVC. In particular, any variation in the present process, such as variation in the amount of the alkaline composition, variation in the treatment conditions (e.g. temperature, etc.), which is different from the process described above for PVC, is within the general knowledge of the skilled person and is therefore not described in detail herein.

As mentioned above, polyester collected from solid waste streams can generally be mixed with aluminum flakes in addition to polyvinyl chloride. The aluminum may come from, for example, the bottle cap of a polyester beverage container, or from incomplete separation of the plastic from the aluminum that may be found in the used waste. Similar to polyvinyl chloride, aluminum is not easily separated from polyester using standard separation methods such as sink/float separation operations.

When contacted with a basic composition and supplied with suitable energy in any or all of the low energy mixer, high energy mixer, and/or heat setting operations, the aluminum can be converted to a basic aluminum salt that is generally soluble in water. Thus, in one embodiment, the amount of alkaline composition that can be added to the polyester and aluminum containing material is sufficient to completely convert the aluminum to an aluminum salt. After the reaction, a fluid, such as water, may be added to the mixture to dissolve the aluminum salt and separate it from the polyester.

According to one embodiment of the invention not all the aluminium contained in the stream entering the reaction stage needs to be converted into aluminium salts. But only a portion of the aluminum can be converted to an aluminum salt by the basic composition. In particular, it has been found that after a portion of the aluminum has reacted with the alkaline composition, the remaining aluminum flakes can become brittle. According to this embodiment, the material containing the polyester mixed with the remaining aluminum may be stirred under shear conditions and the remaining brittle aluminum may be broken into small pieces. These pieces can then be separated from the polyester, for example, by a simple sieving process using a suitable separation operation, such as a screen sufficient to retain the larger polyester pieces while allowing the smaller, broken aluminum pieces to pass through.

Thus, when aluminum is present in the polyester-containing material, the alkaline composition can be added to the material in an amount sufficient to react with at least a portion of the aluminum to render the remaining aluminum brittle. Of course, the actual amount of alkaline composition added may depend not only on the amount of aluminum present in the material but also on the size (e.g., thickness) of the aluminum flakes.

Other operations may be included in the reaction stage, such as additional heating operations, for example, holding the mixture at a suitable temperature for a period of time to further increase the intrinsic viscosity of the product polyester. For example, after a high temperature heat setting operation, such as in a kiln, the mixture is placed in one or more ovens at a desired temperature for a period of time, via, for example, heat conversion and/or radiant heating in the ovens, to ensure the formation of food grade polyesters, to further improve the physical properties of the product, to remove other volatile impurities, and/or to remove other non-volatile impurities, such as paper fibers. In such embodiments, any suitable oven may be used, including an infrared type oven.

Completion stage

The specific operations involved in the finishing stages of the disclosed methods may generally vary depending on the specific impurities and/or impurities (imprints) in the starting composition. Furthermore, depending on the specific impurities found in the starting materials, the present invention can not only effectively recover polyester from various impurities and/or impurities, but also recover specific impurities found in the waste stream as a by-product of the process. The polyolefin removed from the stream in the sink/float operation can be dried and recovered.

In one embodiment, the finishing stages of the process may include at least one washing or rinsing operation, one color sorting operation for removing color-changing impurities from the mixture, and at least one elutriation operation for removing fines and any remaining light impurities from the product.

Optionally, the material is washed without water or, in addition to washing the material with water, the material may be washed with a washing solution. For example, the mixture containing the polyester and any residual impurities may be mixed with a hot aqueous solution containing a surfactant or with a hot aqueous solution containing an alkaline material and washed. If desired, the mixture may be heated with agitation or in a high intensity wash, as described above during the preparatory phase. Cleaning the material may be accomplished by directly separating the polyester from the impurities to clean the polyester and may also dissolve and/or break apart some of the impurities.

During the finishing stage, the acid or metal salts formed during the saponification reaction may be dissolved in the wash water. The metal salt may later be recovered from the wash water for disposal or as a by-product of the process, if desired. For example, if the acid salt is terephthalate, the wash water may first be filtered to remove any undissolved impurities and impurities. The wash water may then be acidified to form a terephthalic acid precipitate. To acidify the solution, a mineral acid such as hydrochloric acid, phosphoric acid or sulfuric acid or an organic acid such as acetic acid or carbonic acid may be added to the solution. Once the terephthalic acid precipitates, the terephthalic acid can be filtered, washed and dried to produce a relatively pure product.

To separate certain impurities, such as certain polyvinyl chloride reaction products, from the polyester, the mixture may be mixed with a fluid, such as water, during the finishing stage. When placed in water, the polyester will sink and the dechlorinated polyvinyl chloride will float. In addition, it has been found that treating polyvinyl chloride with an alkaline composition in the manner described above promotes the attachment of entrained air and other bubbles to the surface of the polyvinyl chloride reaction product, making the polyvinyl chloride reaction product more buoyant. Thus, when the mixture is in a liquid (e.g., water), gas bubbles, such as air bubbles, can be forced into the liquid to increase the separation efficiency. Of course, other separation techniques based on the difference in density of the polyester and the dechlorinated polyvinyl chloride may optionally be incorporated into the present process.

As mentioned above, PVC reacting with alkaline compositions also darkens the color and raises the melting point of polyvinyl chloride. Thus, in another embodiment, the polyvinyl chloride reaction product may be separated from the polyester in the finishing stage via a color sorting operation as described above in the preparation stage. Other impurities may also be separated from the mixture at the finishing stage via a pigment sorting operation. For example, in certain embodiments, the polyester feedstock will contain certain barrier materials, such as acetaldehyde scavenger barrier materials in reacted or unreacted form. According to the invention, such impurities may discolor during the process of the invention, for example during the heat-setting operation of the invention. These impurities, and in particular those which may discolor during the reaction stage of the process, can be removed from the mixture during the finishing stage via a pigment sorting operation.

In one embodiment, the finishing stage may include adding a suitable acid, such as a mineral acid (including, for example, hydrochloric, phosphoric and/or sulfuric acid) or an organic acid (such as acetic and/or carbonic acid), to the mixture to neutralize any residual alkaline material. After the addition of the acid solution to neutralize any residual base, the mixture may be dried prior to any drying separation operation.

During the finishing stage, residual metal impurities are removed from the mixture in a metal removal operation. Figure 2 illustrates one embodiment of a high quality metal removal operation suitable for use in the present invention. Referring to fig. 2, it can be seen that according to this particular embodiment, the metal removal operation may include a plurality of metal detectors in series and parallel combinations to form a bank of metal removal devices to more thoroughly remove metal impurities than previously known metal separation operations.

It should be understood that although this particular metal separation operation is presented herein as occurring during the finishing stage of the disclosed method, equivalent operations may optionally be included elsewhere in the method (e.g., during the preparation stage). Moreover, in the polyester recovery process, the various high quality metal removal operations described herein may be included throughout the various suitable locations of the process.

In one embodiment, the method may include multiple sets of two or more metal removal devices in a single operation. According to this embodiment, the total mixture stream may be split in two or more sets of metal removal units to produce multiple smaller feed streams into each set of units. For example, a single operation step in a polyester recovery process may include one, two or even more sets of equipment. For example, high quality metal removal operations may include from 1 to 10, 20, 30, or even more groups, each group containing two or more metal detection and removal devices.

Referring to fig. 2, a single stack arrangement is illustrated according to one embodiment of the present invention. It will be seen that the group includes 5 individual metal detection and removal devices according to this particular embodiment, but it should be understood that this particular number is not a requirement of the invention. In other embodiments, the set of devices may include additional or optionally fewer individual devices. For example, the set of devices may include as few as 2 devices and does not limit the upper limit of individual devices in the set. In particular, however, determining the preferred number of individual devices comprised in a group and the total number of groups comprised in the operational steps of the method generally involves economic considerations.

All feed streams to the group are processed in the first metal detection cell 1. Upon detection of the metal in the feed stream, a portion of the stream including the detected metal is divided and removed from the stream as rejected stream 10. The "clean" or accepted stream 7 and the rejected stream 10 exit the metal detection unit 1 and enter the other metal detection units 2 and 5, respectively. At the metal detection unit 2, the detection and separation process is repeated, with the rejected stream 11 (including the detected metal) being fed to the metal detection unit 5, and the accepted stream 8 being fed to a further detector and separation unit 3 and then to the last metal detection unit 4. Rejected streams 12 and 13 from metal detection units 3 and 4, respectively, pass into recycle stream 6 and the finally accepted stream 18 continues its entire process. At the metals detection and removal unit 5, the accepted stream 20 is fed back to the recycle stream 6 and the rejected stream 14 is removed from the process as metal-bearing waste material.

In addition to recycling rejected streams 10, 11, 12, 13 and accepted stream 20 from recycle stream 6 in the process, the individual metal detectors contained in the group can be set at increased sensitivity as the streams pass through the process (as indicated by the side arrows). For example, the sensitivities of the metal detecting units 2, 3, and 4 may be sequentially increased as compared to the metal detecting unit 1. Also, the sensitivity of the metal detection unit 5 may be higher than that of the metal detection unit 1.

By the process of the present invention, including high quality metal removal operations, substantially all of the metals contained in the feed stream to the process can be removed from the polyester product stream.

Typically, the finishing stage of the operation may also include one or more elutriation operations to remove any remaining light impurities such as any remaining paper or paper fibres or any polymer fines.

Other separation operations that may be used in the finishing stage and between the operations of the preparation and/or reaction stages may include physical separation operations such as those described above in relation to the preparation stage, as well as any other suitable separation operations known in the art, such as the use of one or more de-agglomeration operations to remove glass, and/or the use of hydrocyclones. For example, in one embodiment, a dual hydrocyclone scheme may be used in which material is pumped to a first hydrocyclone to remove high density contaminants such as glass and/or metal, then to a second water tank and pumped to a second hydrocyclone to remove low density contaminants such as paper and/or polyolefin.

One or more vibratory screening methods may also be used at various locations throughout the disclosed process, for example to separate contaminant material exiting the top of the sink/float tank from the process water for removal of contaminants in powder form after the heat setting operation, after the high shear washing operation of the preparation and/or finishing stages, or after the final washing and drying operation of the finishing stages.

In one embodiment, a flushing operation may be used to remove heavy impurities from the mixture at any convenient point in the process. For example, it may be advantageous in certain embodiments of the process to include a flushing operation to remove certain heavy impurities from the mixture, either just before or just after any aqueous separation operation in the preparation stage, or just before or just after any aqueous separation operation in the finishing stage.

The treatment process of the disclosed invention can provide a number of significant advantages. For example, it can clean and/or remove impurities from the polyester. In fact, the polyester can be cleaned and/or impurities removed from the polyester to a level sufficient to comply with various regulations imposed, and in particular, a food grade polyester product can be produced. Of course, it should be recognized that the target level of cleaning and/or removal of impurities may depend on the end use application of the polyester. In particular, the present process can provide recycled polyester products with improved properties, such as high cleanliness, good color and even improved intrinsic viscosity. Moreover, the disclosed processes can provide these products in acceptable yields and with lower processing costs, e.g., without the need for "repolymerization" of the monomers as compared to conventional depolymerization processes.

Such modifications and variations that may be apparent to a person skilled in the art may be made to the present invention without departing from the spirit and scope of the invention as specifically set forth in the appended claims. Further, it should be understood that aspects of the embodiments may be interchanged both in whole or in part. Furthermore, those skilled in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in such appended claims.

Claims (18)

1. A method for separating polyester from impurities comprising:

providing a mixture comprising impurities and a polyester;

removing a portion of the impurities from said mixture in an elutriation operation, said mixture being fed to said elutriation operation at a flow rate of incoming solids of 2500-3500 lbs/hr, said elutriation operation comprising a flow rate of air of 3600-4600 cubic feet/min, wherein impurities heavier than the polyester are removed during the elutriation operation and the lighter stream separated during said elutriation operation comprises said polyester;

mixing the mixture with an alkaline composition after the elutriation operation; and

only a part of the polyester is saponified according to a saponification reaction between the alkaline composition and the polyester.

2. The method of claim 1, wherein said alkaline composition is an aqueous composition and said mixture is mixed with said alkaline composition to form a slurry.

3. The method of claim 2, further comprising mixing said slurry in a high energy mixer, wherein said mixing imparts sufficient energy to said slurry to promote a saponification reaction between said polyester and said alkaline composition, and wherein said saponification occurs in said high energy mixer in accordance with said saponification reaction, wherein any heat added to said slurry in said high energy mixer is insufficient to independently promote said saponification reaction between said polyester and said alkaline composition.

4. The process of claim 1 further comprising heating said mixture to a temperature no greater than the melting point of said polyester in a heat setting operation wherein said mixture is heated in an environment containing less than 80ppm water content.

5. A process for separating polyester from impurities comprising

Providing a mixture comprising impurities and a polyester, wherein the impurities comprise a white polyester colored with titanium dioxide;

subjecting the mixture to a color sorting operation, wherein the white polyester is removed from the mixture during the color sorting operation, leaving residual polyester in the mixture;

mixing the mixture with an alkaline composition after the colorant picking operation; and

saponifying only a portion of the remaining polyester according to a saponification reaction between the alkaline composition and the polyester.

6. The method of claim 5, wherein said alkaline composition is an aqueous composition and said mixture is combined with said alkaline composition to form a slurry.

7. The method of claim 6, further comprising mixing said slurry in a high energy mixer, wherein said mixing imparts sufficient energy to said slurry to promote a saponification reaction between said polyester and said alkaline composition, and wherein said saponification occurs in said high energy mixer in accordance with said saponification reaction, wherein any heat added to said slurry in said high energy mixer is insufficient to independently promote a saponification reaction between said polyester and said alkaline composition.

8. The process of claim 5 further comprising heating said mixture to a temperature no greater than the melting point of said polyester in a heat setting operation wherein said mixture is heated in an environment containing less than 80ppm water content.

9. The method of claim 5, further comprising subjecting said mixture to a second color sorting operation after saponification.

10. A process for separating polyester from impurities comprising

Providing a mixture comprising impurities and a polyester, wherein the impurities comprise aluminum;

combining the mixture with an alkaline composition in a low energy mixer to form a slurry;

reacting the alkaline composition with at least a portion of the aluminum while the mixture is in the low energy mixer;

mixing said slurry in a high energy mixer, wherein said high energy mixing imparts sufficient energy to said slurry to promote a saponification reaction between said polyester and said alkaline composition; and

saponifying only a portion of said polyester in said high energy mixer in accordance with said saponification reaction, wherein any heat added to said slurry in said high energy mixer is insufficient to independently promote the saponification reaction between said polyester and said alkaline composition.

11. The process of claim 10 further comprising heating said mixture to a temperature no greater than the melting point of said polyester in a heat setting operation wherein said mixture is heated in an environment containing less than 80ppm water content.

12. The method of claim 10, further comprising flushing the slurry after at least a portion of the aluminum has reacted and adding additional alkaline composition to the slurry prior to saponification in the high energy mixer.

13. A process for separating polyester from impurities comprising

Providing a mixture comprising an impurity and a polyester, wherein the impurity comprises a metal;

removing at least a portion of the metal impurities from the mixture in at least one metal detection and removal operation, wherein the at least one metal detection and removal operation comprises at least one set of metal detectors, each set comprising two or more metal detectors in series combination, in parallel combination, or in both series and parallel combinations;

mixing the mixture with an alkaline composition; and

only a part of the polyester is saponified according to a saponification reaction between the alkaline composition and the polyester.

14. The method of claim 13, wherein said alkaline composition is an aqueous composition and said mixture is mixed with said alkaline composition to form a slurry.

15. The method of claim 14, further comprising mixing said slurry in a high energy mixer, wherein said mixing imparts sufficient energy to said slurry to promote a saponification reaction between said polyester and said alkaline composition, and wherein said saponification occurs in said high energy mixer in accordance with said saponification reaction, wherein any heat added to said slurry in said high energy mixer is insufficient to independently promote a saponification reaction between said polyester and said alkaline composition.

16. The method of claim 13, further comprising heating said mixture to a temperature no greater than the melting point of said polyester.

17. The method of claim 13, wherein the at least one metal detection and removal operation comprises forming an accepted stream and a rejected stream at each metal detector, the method further comprising recycling at least one rejected stream through the metal detection and removal operation.

18. The method of claim 13, wherein the at least one set of metal detectors comprises at least two metal detectors in series, wherein the sensitivity of the series of metal detectors to metal increases with the number of series stages.