CBOT sampling protocols

Best Practices in PET Recycling

Granulating

Issue: Properly designed and maintained PET granulating systems will optimize material quality, production efficiency and throughput, and general workplace safety.

Best Practice: To recap, the first best practice is to make sure that the quality considerations detailed in the Introduction and Contamination Issues, PET Regrind Specifications and Sorting sections of this document are adhered to when grinding PET bottles and containers for sale to reclaimers or end-users.

There is no best piece of granulating equipment that can be recommended for use at PET intermediate processors. There are many types, sizes and manufacturers of granulators, or grinders, and regrind evacuation systems currently available that provide quality grinding capacity. Grinders come in many design configurations in terms of infeed size and type, motor size, grinding chamber size, the type of cut performed, etc. The specific choice of granulation system for a facility is a function of program budget, anticipated throughput, overall system design and plant layout. However, there are some basic grinder design types that should be specified when purchasing grinders for your facility and some best practices for their proper maintenance and operation in your facility’s grinding operation.

Granulators operate as follows. They generally contain two sets of cutting blades. One set is fixed. The fixed blades are known as bed blades. The other set is mounted on a spinning rotor that cuts PET bottles against the bed blades as material is fed into the cutting chamber. These are called the rotor blades. The number of bed and rotor blades used in grinders varies with the specific equipment. At the bottom of the cutting chamber is a metal “screen” with a specific hole size. When PET bottles are cut small enough, the granulated material will fall through the holes in the screen and enter a regrind evacuation system. A blower removes ground PET from the grinder and blows it though ducts to a cyclone, from which it is discharged into boxes.

Based on a survey of PET plastic grinding facility operators, the consensus is that the best grinder design is grinders with open rotors, that are tangentially fed and perform a “scissor-cut” of bottles entering the cutting chamber. The best cutting blades for these systems are known as “back-angle” knives. It should be noted that this grinder design works best for whole bottles at intermediate processing capacities of less than 3,000 pounds per hour. PET plastic reclaimers and end-users that require greater processing capacity use completely different grinder types that are specially designed for larger processing volumes. These include “hog” grinders and wet grinding systems.

In addition to the grinder itself, granulating systems also consist of the regrind evacuation system which includes a blower, ductwork, and cyclone, where regrind is removed from the grinder and blown through the cyclone, which then discharges the regrind into a box, while venting small dust particles through an exhaust port (these small dust particles are known throughout the industry as fines, and are generally undesirable). It is very important when purchasing a granulation system to work closely with the equipment manufacturer or sales representative, and to provide adequate information to ensure that the grinder and evacuation system you purchase are properly and adequately sized to handle the anticipated throughput of PET plastics that will be processed and are consistent with overall facility design. In addition, it is important to specify to a manufacturer whether your facility will be grinding predominantly flattened, partially flattened bottles (i.e, from bales), or unflattened, whole bottles. Generally, grinders will have higher throughput when bottles are flattened. However, the volume and nature of the PET bottles fed to a grinder will impact such items as the entry feed, or “throat,” size.

Once a system has been selected, there are a number of installation best practices that will ensure safe operations of that equipment (and may be regulatory requirements). The first is that all grinding equipment and the conveyors that feed them should be equipped with emergency shut-off switches that will cut power to the system in an emergency or safety hazard situation, or when maintenance is required. Second, all employees feeding grinders should be trained in their proper operation. Third, grinders and regrind evacuation systems should be soundproofed to maintain noise levels within regulatory worker noise exposure limits. Finally, all cyclones should be properly vented and exhausted in a fashion consistent with applicable regulatory requirements. For example, applicable regulations may require the use of baghouse filters, or other dust collection systems.

There are several maintenance best practices that can greatly increase the performance and material throughput in your grinding system, and can ensure the quality and marketability of the regrind your facility produces. While trained grinder operators can usually hear or see when equipment needs maintenance, every facility should have a schedule of regular and preventive maintenance performed on their granulation system equipment.

Proper blade maintenance is crucial to material quality. The distance between the rotor and bed blades in a grinder are gapped to very small tolerances that must be maintained for efficient cutting. This gap distance may change during normal operation. In addition, blades need to be kept sharp for maximum operating efficiency. Proper blade sharpness and gapping are essential to produce quality regrind. Trained operators can tell from the quality of the regrind produced when blades are beginning to dull. This can usually be identified through the detection of excessive amounts of fines, or plastic dust, or regrind particles that do not have a “clean” cut. It is a best practice to have a full, sharpened set of extra blades for each grinder in operation. When the blades are dull, the extra set can be installed and the dull set sent out for sharpening. It should be noted that blade life between sharpenings can be extended by re-gapping.

Maintaining clean screens in grinders will also maintain optimal material throughput. The size of the screen used in your grinder will depend on your particular regrind purchaser’s specifications, however, 3/8" screens are probably the most common. If screens are clogged, plastics will remain in the cutting chamber of the grinder longer. When this occurs, the heat build-up in the cutting chamber can melt plastics to the screen, further clogging them. A best practice is to clean screens with a wire brush at the end of each shift. If the screen is excessively clogged it should be replaced while it is cleaned.

Another maintenance best practice is to keep a supply of spare parts for items that commonly fail in grinding systems, including, but not limited to, spare screens, heater coils, belts, and even starter motors. This will facilitate in-plant repairs and lessen downtime on equipment.

Another best practice is to perform all equipment maintenance “off-shift” so that production is not impacted. Finally, grinders should never be turned off with materials still in the cutting chamber, unless in an emergency. If too much ungranulated material is in the cutting chamber of a grinder when it is turned on, electrical failure is common, as it will impede proper movement of the rotor. If a grinder must be stopped for any reason while the cutting chamber is full, the cutting chamber must be cleared of all material prior to resuming operation.

One last best practice in PET plastic granulating is to properly mark or label each box of regrind produced, so that if a processing problem occurs at a reclaimer’s or end-user’s facility, the exact conditions of the regrind manufacture can be traced back. This is often helpful in resolving costly disputes regarding contaminated materials and the financial costs they can impose. At a minimum each box should be marked, or labeled, in such a way as to identify the operator who prepared the material, the particular equipment it was ground in, the date it was produced, as well as the full, tare, and net weights of the box. If PVC test results are available, they should also be indicated.