Garry R. Kenny, Mitchell G. Roe, Felix A. Hottenstein
Identiplast '99, April 26-28, 1999, Brussels, Belgium

Abstract

Sensors for identification of recycle plastics must be integrated into automated sorting systems before becoming commercially viable equipment for the recycling facility. This paper provides a brief history of sensor and system development, an overview of sensor integration into operating systems, and discusses new applications and developments in available sensors and systems for separation of plastics for recycling.

Background

The first commercial sensor (ca. 1991) available for separation of recycle plastics was specific to PVC identification, and relied on absorption of x-rays by Chlorine atoms present in PVC bottles. Work by Rutgers University at the same time focused on a specific sensor utilizing visible light for identification of colored from clear PET, while using a separate x-ray fluorescence based sensor for PVC identification.

Concurrently, MSS developed and introduced a single frequency near infrared-based (NIR) sensor, which was commercially successful for separation of PET, natural HDPE and pigmented HDPE in 39 sorting lines worldwide. Separation of clear from colored PET utilized a separate sensor specific to this separation (40 sorting lines), as well as an x-ray based sensor for PVC from PET separation (71 sorting lines).

The practice of using single sensors for specific resin or color separation continued until 1994 when Buhler. Ltd. of Switzerland introduced the NIRVIS™ sensor which utilized a single channel, multi-frequency NIR sensor with the capability of identifying most of the majority consumer plastic packaging resins (codes one through seven). The NIRVIS™ sensor was developed by OPTAN, also of Switzerland. In 1995 Binder & Co. of Austria introduced the Criterion™ sorting system which utilized another single channel, multi-frequency, NIR sensor provided by LLA GmbH of Germany, as well as a color sensor provided by Massen Machine Vision systems GmbH, also of Germany.

Both of these NIR sensors utilize a single identification channel, requiring that the plastic bottles to be identified be fed to the sensor one by one (singulated), and within a limited sensing width. Development of this type of multi-frequency, single channel type sensor has continued since this time, numerous examples of which were reported at the 1997 Identiplast 1 conference.

In 1996 MSS introduced a multi-channel, multi-frequency NIR sensor system for identification of either plastic resin type or color. This multi-channel sensor operates in both the near infrared and visible light range, providing either resin or color identification. This multi-channel sensor, designated PSI™ for Parallel Spectral Identifier, was developed to overcome the limitations of single channel, singulated feeding sensors as well as feed width limitations.

In 1996 TiTech of Norway introduced their AutoSort system which utilized a single multi-channel sensor, but implemented with a novel mechanical scanning mechanism. The combination of the single sensor with the mechanical scanning mechanism provides an output that is similar to MSS’ multi-channel approach, albeit limited to the scan rate of the mechanical system.

Material Preparation & Sensor Integration

Proper integration of the sensor into the mechanical separation unit, and the unit into the overall separation system is critical to success of a sorting facility. Experience in development and operation of plastic separation systems, whether in bottle or granulated form, shows that proper feeding and proper preparation of the feedstream is critical to optimal separation efficiency of the material and at least equally important as the quality of the sensing system used.

For baled bottles, a mechanical system for reducing plastic bales into single bottles is critical. Debaling systems should include a secondary declumping unit for reducing bottle clusters from debalers to single bottles, or be followed by a stand-alone declumper.

Feed preparation should also include a mechanical system for screening of plastic bottles to remove caps, fines, and other contaminants. Plastic bottles derived from mixed recyclable collection systems such as DSD in Germany should provide for removal of oversize objects, films via air separation or manual removal systems, and screening of undersize materials. Bottles also need to be sufficiently flattened in order not to roll or change position on high-speed conveyor belts.

It is desirable to be able to apply only one single sensor type to identification and separation of all plastic resin types and colors. However, it is MSS’ experience, that applying different sensor types to specific identification needs generally provides better separation. The primary consideration is to apply the proper sensor, or sensors, to the specific application in order to obtain the best available separation efficiency, with the highest reliability, and at the least cost. In making this judgment, it is also necessary to consider how the plastic bottle stream mixture or collection system may change in the future.

Plastics Feeding Systems

There are, in general, two methods of feeding: 1) Singulated feed, and 2) mass or random feed. In the singulated feed method, plastic objects are fed to a sensor and subsequent ejection system in a one-by-one manner. The primary advantage is that after passing through a single sensor system, multiple resin separations of the bottle stream can then be made. Singulating system, however, have the limitation that if multiple objects pass through the sensor at one time either an identification error or an ejection error will be made. Singulated systems also require relatively complicated and space intensive feed systems, and have a feedrate limitation of 550 to 680 kg/hr (1,250 to 1,500 lbs/hr).

In the mass or random feed method, multiple plastic objects may be fed to the separation system at one time. Since several objects may be within the sensor region simultaneously, either independent electronic channels or sophisticated electronics with the capability to identify and track individual bottles must be employed to provide accurate identification and ejection.

Mass fed systems have higher unit feedrates than singulated systems and require less sophisticated feed systems. Typical mass fed separation units have feedrates of 1.100 to 2.200 kg/hr (2,500 to 5,000 lbs/hr).

The Recycle Plastic Market

A fundamental change in plastic bottle recycling has occurred during the last two years – worldwide more plastic bottles are now sorted using automated equipment than are being sorted by manual means. The ratio of automated to manual sorting varies from country to country, and the use of automated equipment is increasing in countries where manual sorting is in the majority.

One of the current needs of plastics recycling is an increase in the availability of post consumer plastics. Due to the increased usage of plastics in packaging applications, particularly PET, the actual percentage of plastics recycling has declined in several countries, most notably the United States. Significant price swings over the past few years has contributed to a reduction in the number of cities collecting recyclable materials. Additionally, a general decrease in public interest in recycling has also contributed to a slowing in the rate of increase of recyclable plastics collection.

New Municipal Waste Sorting Applications

Recovery of recyclable metal, glass, and paper from municipal waste and collection systems has been practiced for a decades, but plastics recovery has been limited to mixed recyclable collection systems. Plastics recycling, with a more restricted supply source, are at greater risk from market downturns than the above materials as the recycling infrastructure is not as mature.

A potential new source is the recovery of recyclable plastics from mixed municipal waste. Such a facility has been placed in operation since July 1998 in the Crisp County Facility in Cordele, Georgia, USA. It is located in a rural area and provides collection to 21 different counties. The member counties are relatively sparsely populated and as such do not provide curbside collection of recyclable materials.

The facility has a processing capacity of 1,000 tons of municipal waste per day and uses a combination of separation equipment to process raw, mixed municipal waste into recyclable components. The process includes, trommel screens, magnetic separators for ferrous metal, eddy current separators for nonferrous metal, and an automated system for separation of plastic containers. The automated plastic separation equipment is located at the end of the raw waste process line. An extensive composting process is also integral to the facility.

The recovery of post consumer plastic containers from mixed municipal waste represents a new source of recycled plastics. The plastic separation equipment recovers an average of 40 tons per day or 14.000 tons (15,400 t/yr) per year of plastics. The plastics separation system was designed by MSS to separate the plastic into five components. The separation system has passed performance tests with respect to the designed capacity of 2.200 kg/hr (5,000 lbs/hr) while providing plastic purities within specifications.

The installation of ten Crisp county type installations would increase the amount of post consumer plastics available for recycling by nearly 30 percent in the U.S. alone. In addition, installation of Crisp county type facilities are being considered in a number of countries where relatively low population densities favor this approach to municipal waste processing with recyclable recovery.

New MRF Sorting Applications

The primary supply of recycled plastics to the reclaimers in the U.S., France, Germany, Canada, and several other countries are Material Recycling Facilities (MRF’s). These facilities function to accept mixed recyclables collected at the curbside or deposited in dropoff boxes. The function of MRF’s are to provide the basic separations required to add value to the individual recyclable components such as ferrous and nonferrous metals, plastics, paper and cardboard.

The degree to which the plastics are sorted by resin and/or color type depends upon the individual MRF. Additionally, the number of manual sorters required is higher for non-automated MRF’s than would be required if automated equipment were utilized. For example, in the Alliance Waste facility in Schaumburg, Illinois, USA the number of persons required to separate the plastic waste was reduced, on average, by 5 persons after an MSS automated plastic sorting system was installed. The resulting cost savings were such that the automated equipment payback time was less than sixteen months.

Widespread introduction of automated plastics separation equipment into MRFs would decrease facility operating costs while providing a more consistent quality of separated plastics to the end processors (PRFs – Plastics Recycling Facilities). Germany and the U.S. each currently have approximately 200 MRFs that could benefit from installation of automated plastics separation equipment.

New DSD Sorting Plants applications

The separation cost of plastic materials in the German DSD system is currently relatively high. A significant portion of this cost is due to the high cost of manual sorting. To reduce these costs, automated sorting modules have been introduced to the German DSD sorting plants.

Two basic types of systems are currently available for this application was: 1) sort all plastics away from other materials, and 2) sort aseptic packaging away from other materials. Over the past two years approximately 75 such systems have been installed in DSD plants. The labor saving due to these installations has been significant, and additional automation will further reduce cost.

Two companies currently provide separation modules applicable to DSD sorting plant applications, MSS and TiTech. The MSS systems are provided under the trade names of PlasticSort™ and CartonSort™ respectively. The TiTech systems are provided under the name of AutoSort GTÔ and AutoSort KT™. Both systems utilize reflective NIR sensors for identification of plastic and aseptic packaging materials, and as such are able to be positioned over existing or new conveyor systems.

The MSS systems utilize the PSI sensor previously discussed. Each channel of the PSI sensor array is an independent NIR sensor allowing simultaneous processing of the acquired NIR data from all channels. The processed data is then examined by a master microprocessor to determine the object shape, position and resin type. The parallel processing method utilized in the PSI sensor allows for very high-speed acquisition of data from the sensed region, as opposed to scanning of the sensed region with a single channel NIR sensor.

After identification, the selected material is ejected either downward or upward perpendicular to the flow of the feed stream by a conveyor wide array of air jet ejectors. The systems have a capacity of between 2.000 and 2.500 kg/hr (4,400 to 5,500 lbs/hr) with a sorting accuracy of 80% to 90% depending upon the feedstream and the material removed.

New Flexible Resin Separation Systems

In MSS’ experience, sensors for identification of recycle plastic which attempt to identify multiple resin types have limited accuracy compared to sensors dedicated to one, two, and at the most three resin identifications. The vision of an ‘all-in-one’ system, which has the capability to perform all desired sorts with one sensor, while providing high accuracy at a reasonable cost, is yet to be commercially demonstrated. As discussed above, available systems have been limited to either low capacity singulated feed systems, mass feed systems with pre-set sorting capability or, multi-resin, single channel sensor systems with limited accuracy.

To address these constraints, MSS has developed a new sorting module, designated the B1600 which utilizes the PSI sensor integrated into a 1.6 m (64 inch) wide feed system to provide both high separation accuracy and resin sorting flexibility in a mass feed format. The B1600 system combines the advantages of the singulated multiple resin systems with the feedrate or mass fed systems while avoiding the accuracy problem of single channel, multi-resin sensor systems.

The sorting module utilizes the PSI sensor system, which consists of multi-channel, multi-frequency sensors based on the newest available 32 bit industrial microprocessors. The combination of the PSI sensor with the integral feed system allows the operator to choose and/or change a up to three sorting combinations to his desired needs within seconds.

Complete flexibility to select which sorts are to be provided (such as PAN and PEN) can be accomplished in less than a hour via a mechanical change. The B1600 module has been specifically developed for high user flexibility to address the changing material input and desired output of plastic bottles or waste streams. The B1600 module has a capacity of up to 5.540 kg/h (10,000 lbs/h).

Real Vision Systems GmbH of Germany has presented similar research work (partially funded by DSD) in early 19983) but no commercial field installation has been reported as of late 1998.

New Resin Identification (PEN, PAN)

A sensor system has been specifically designed by MSS to identify PEN (Polyethylene Naphthalate) and three different PEN/PET blends in a stream of PET bottles. The sensor development was partially funded by AMOCO and Shell Chemical, who recognized that recycling of PET bottles was a large and viable industry which could potentially be impacted by the introduction of PEN bottles.

This support is noteworthy in that it was provide before introduction of PEN to the market, thereby insuring that a system for PEN separation was available when it was needed. With the strong intention of the beer industry to provide beer in plastic bottle packagings (instead of glass) this issue could be of significance to plastics recyclers in the near future2).

Two generations of the MSS PEN sensor systems were extensively field-tested at the Wellman recycling facility in Johnsonville S.C. Wellman provided the equipment and support to expose the PEN test containers to the typical recycle conditions of baling, debaling, and materials handling. The test PEN containers were also provided with simulated labels.

The results of the testing of the PEN separation system shows an identification accuracy of between 92% and 99% depending upon the type of bottle tested. The presence of food contamination or labels reduced the system sorting accuracy’s by only a relatively small amount.

MSS recently completed work on the development of a sensor system for identification of PAN (Polyacrylonitrile) plastic bottles. This work has been partially funded by BP Chemical. Laboratory testing of the system shows an identification accuracy of 92% to 98% for test bottles. Both the PEN and PAN separation systems, utilizing the developed sensor, are now commercially available.

New Flake Analysis Systems (PVC, PEN)

The ability to identify and separate plastic resin in a flake or granulated form has the potential to save plastics processors significant costs, both in processing and quality control costs. MSS recently completed a study for an analysis system that can quantify the amount of PEN plastic present in PET flake. The American Plastics Council, under the direction of the Naphthalate Stewardship Council sponsored the study.

Work is currently underway by MSS to develop a system to identify PVC content in parts per million of PVC in granulated PET. Applications include batch testing of processed PET as well as real time monitoring of PVC levels at rates of 25 to 30 kg/hr.

Conclusions

The use of automated plastics separation systems continues to increase worldwide. New sensor technologies continue to be introduced, but the fundamentals of proper feed preparation, feed methods and proper sensor application continue to be important. New applications of automated separation equipment to MRF and DSD type application is increasing, and sensor technology is being applied to newly developed plastic resins such as PEN and PAN. Developments in plastic flake analysis promises lower cost quality control and higher quality flake in the future.

References

1. Proceedings Identiplast Conference 1997, Brussels, Belgium
2. Proceedings ARC Conference 1998, Chicago, USA
3. Proceedings 3. DKR Congress 1998, Bad Neuenahr, Germany