Technology Notebook: Recycling PET postconsumer bottles

Uses for recycled PET are expanding, both domestically and abroad. At the time of this writing, recycled PET clear flake is selling for 44 cents/lb on average, while green flake is 38 cents/lb average. To profit at those price levels, recyclers must operate efficiently. A rotary knife mill (granulator) is an effective means to reduce PET scrap into nominal 3?8-inch flake, enabling it to enter downstream equipment where clean flake, pellets, or filament is ultimately produced into a variety of consumer products labeled with their percentage of recycled content. In the majority of cases, the recycler requires the granulator to maintain a 4000 lb/hr average production rate over a 16-hr day, 22 days/month, resulting in 1.4-million lb processed, which equates to a staggering volume of 11 million 2-liter beverage bottles (8 bottles/lb).

Typical granulators are subject to premature wear of vital components i.e. blades, bed-knife seats, knife hold-down bolts, classifying screen(s), and the walls of the cutting chamber, essentially all surfaces coming in contact with the abrasive PET scrap. Replacing these components is costly and time consuming. Preventive measures to extend these components? lives include using blades with a dense grain structure or carbide impregnation; replaceable wear plates covering the walls of the cutting chamber; replaceable bed-knife seats; wear-resistant screen(s), or introduction of water into the granulator, known as wet grinding. With water flushing, pneumatic evacuation systems are not required. All metal contaminants must be removed from the PET prior to its entering the granulator. The cleanliness of the bottles also plays an important roll in the life expectancy of the granulator?s internal components. Bottles collected in plastic bags from beverage companies are usually much cleaner, with fewer contamination than those collected in bales from transfer stations. Baled, crushed bottles are more dense than whole bottles, allowing higher granulator output.

Output capability and access to the inner workings of the machine are the most important granulator characteristics. Production output might be projected by the extent of experience the granulator supplier has with current PET customers and whether or not he can provide all necessary anciliary equipment, or has the expertise to recommend the proper equipment. One- source supply can ease some stress on the decision maker. Maintenance issues, i.e. accessibility to the cutting blades via clamshell opening of the cutting chamber to expose the rotor and primary and secondary bed knives can be simplified, with minimal labor involvement, if the granulator is equipped with an electrically-activated pushbutton-controlled hydraulic cylinder to open and close the upper portion of the cutting chamber. In addition, the screen cradle is lowered and raised via another pushbutton-controlled hydraulic cylinder. The type of steel used to fabricate the cutting blades is critical, as is easy access to replace them. Double-angle cutting action is also essential. A true scissors cutting action created when the rotating knives are counter-angled to the bed knives is fundamental to a uniform regrind and minimizing production of fines. Some granulator suppliers angle the rotating knives but keep the bed knives straight, which we do not favor, because it can result in tearing or pulling the PET scrap apart creating fines that disrupt the effectiveness of the downstream equipment. Another critical consideration is the distance between the rotating knives and the screen surface. When the rotating knives are sharpened, the distance between rotating knives and the screen?s surface increases accordingly, causing the flake to straddle the screen and not be pushed through by the knives. This results in reduced throughput, increased amperage draw, and over-cutting the scrap into a less desirable particle size. A solution is to make the rotating knives adjustable, thus creating a constant cutting circle dimension. Adjusting the knives while attached to the rotor is not only time consuming but dangerous, so it?s recommended that a knife setting fixture or jig be provided, allowing the replacement set of rotor and bed knives to be prepared for installation by setting the gap between them outside the granulator while it?s still in operation. This method will reduce knife changeover time and minimize machine down time because the dull knives need only to be removed and the fresh knives torqued into place.

Sorting clear bottles from green bottles prior to or after granulation is generally a decision the customer makes depending on his preference and available cash. Commercially available optical sensors can separate whole bottles before entering the granulator or separate the mixed flake after granulation. Sorting prior to granulation requires a granulator to be dedicated to the clear bottles and another dedicated to the green. The granulator for green bottles can be smaller because usually the production volume is less then the clear. Sorting after granulation only requires one large granulator.

Wet vs. Dry Granulation: advantages of flushing water through the granulator

Possible Disadvantages: