The Case for Wet Grinding

In the ongoing debate over how best to process recycled post-consumer plastics, a convincing case can be made for wet grinding.

Choosing the right size-reduction equipment for a particular recycling plant can be the key to efficiency and long-term success. There are other important choices, to be sure. Plastics recycling has become more complicated in light of increasing volumes, waste-stream contamination, new sorting procedures, automation, newly accepted types of plastics, etc. Depending on their customers, plants break down different scrap plastics streams into particles of varying size and purity. But at the heart of most operations is shredding, granulating or pulverizing.

FEED RATE

Critical to any recycling business is feed-rate optimization. Input has to be fast enough to keep up with desired output, but, more importantly, it should be steady.

For recycled milk jugs, beverage bottles, yogurt cups, trays from heat-and-eat meals and a multitude of other household containers, horizontal screw-fed systems generally work better than gravity-fed systems. They can granulate polyethylene terephthalate (PET) and other plastics at rates of up to 7 tons per hour—a throughput than can be 30 to 50 percent higher than same-size gravity-fed systems.

Horizontal feeding screws are designed to force material from a hopper to an adjacent cutting chamber at a constant, even pace instead of depending on gravity to direct the material into the blades. Other benefits can include reduced power consumption (because of a more consistent electrical load), quieter operation, no fly-back, reduced wear to the cutting blades and reduced fines and dust (becuase granulated particles spend less time in the cutting chamber).

The best machines are suitable for dry or wet operation and also can be used for secondary size reduction of pre-shredded plastics, but there are plenty of good reasons for grinding with water. After a few hours of dry cutting, the blades in a typical cutting chamber get hot enough to melt plastic and contaminants (paper labels, adhesives, dirt, food residues, etc.) together, which makes subsequent cleaning much more difficult. Processing with water keeps the cutting chamber cooler, and the friction in the cutting chamber combines with the water to double as an intense washing process.

A properly designed wet granulator will feature both cutting chamber and rotor journal seals to keep the water inside the machine, not on your floor. A water-injection manifold with valves and nozzles introduces the water into the cutting area.

Additional washing and separation of contaminates from the granulated particles can be done by means of a friction washer or dewatering screw.

CUTTING EDGE

Wet grinding also can increase blade life, because blades in a wet grinder generally can go about three times longer between resharpenings than blades in a dry grinder. The additional blade life is a result of washing and flushing the contaminants (dirt, sand, etc.) from the product before granulating. These benefits also can carry over to the machine’s internals and sizing screen.

Everyone knows that operating costs and uptime when recycling plastics determine profitability. What we tend to forget when buying size-reduction equipment is that the least expensive machine may not be the best overall value, especially when it comes to maintenance issues. Ask your supplier what type of knife is being installed (material, hardness, edge type), how much regrind surface is available, how the knives are accessed and adjusted, etc. The answers to these simple questions can add up to big savings in the long run.

The knives of some machines can be removed easily and safely and adjusted in a fixture outside of the unit. This design can allow for more precise adjustment of the cutters, resulting in optimal size reduction, higher throughput and reduced downtime.

At the risk of stating the obvious, the design of the cutting system is important. Tangential cutting chambers, into which material enters from below and behind, are usually a better choice over drop-through chambers. Open rotors tend to operate more quietly with less heat generation and can be better for bulky items or items with shape.

Adjustable rotor knives also can be a better choice, because screen clearance remains the same and they are designed to offer more sharpenings than non-adjustable knives.

Speaking of screens, do not underestimate the value of the flexibility that comes with a granulator versus a shredder, which allows interchangeable screens to control particle size. Machines that accept a wide range of screens can produce fragments from 2 to 3 inches all the way down to 1/8-inch particles.

A diagonally split housing that allows complete access to the cutting chambers can make maintenance faster. Some equipment manufacturers position the rotor bearings outside the cutting chamber to completely separate the chamber from water, dirt and fines. Outboard-mounted bearings also keep grease from entering the cutting chamber and contaminating product.

Last but certainly not least among cutting-system considerations is the machine’s cutting geometry. We strongly recommend double cross-cutting action—the true scissor cut. In this design, the rotor and bed knives are fitted diagonally opposed to each other. This is designed to ensure a constant cutting gap along the full length of the cutter, low temperature increases during granulation and minimal power consumption. This blade geometry, in combination with an open rotor, can provide cleaner cuts, higher capacity, fewer fines and less noise.

CLEAN STREAMS

Sooner or later, someone is going to have to wash the contamination off the plastic you are processing. Why not you? Clean streams can save on maintenance costs by minimizing wear and tear to your machinery. Metal, for instance, can damage cutting blades. In addition to debalers, shaker tables, vibro-troughs, various types of metal detectors and other sorting and pre-wash equipment, there is an array of wash line equipment available for any and every plastics recycling application. They can be used together for new plants or separately to replace or complement machines at existing plants.

For example, friction washers typically receive a slurry of plastic fragments and water from a wet grinder/granulator, but they are sometimes installed after separation tanks. Fed from the lower end of its inclined cylindrical trough, the machine makes use of a fast-running paddle screw surrounded by a screen/sieve to dewater and clean the fragments as they are transported to the outlet at the top of the trough.

Separation tanks separate floating materials from sinking materials. Fed by turbo washers and operating under pressure, hydrocyclones, often used in recycling PET bottles, can accomplish the same task more effectively, especially when used in combination with a separation tank. Floating materials leave at the upper end of a hydrocyclone with most of the water, while sinking materials are expelled from the lower end. This effect can be adjusted by changing the sizes of the outlet nozzles.

Mechanical dryers centrifugally dry plastic fragments while spinning off some remaining contaminants, but they also can be used as intensive washers. The washing version uses a high-speed agitator and a high volume of water to remove paper and other contaminants. Thermal dryers provide additional drying when necessary.

Finally, no matter what your business is today, your customers could change in the future, so look for machinery that is adaptable, with a variety of options and easily-attached accessories. You should also ask around to make sure that the equipment provider has a dependable track record with its customers for rapid service and parts availability.

David LeFrancois is president of Herbold Meckesheim USA, a subsidiary of Herbold Meckesheim Germany that designs, makes and installs size-reduction equipment and wash line systems for the plastics industry.