VII.  WASHING

Washing HDPE Flake

ISSUE:  Washing, rinsing, and gravity separation are key steps in purifying HDPE flake.  Not only are soluble contaminants removed, but glues, adhesives, and other non-soluble contaminants are separated from the flake.  Proper design and operation of the washing step are essential to achieve necessary quality and desired quantity of production.  Care must also be taken so hostile materials, like bottles with “bleeding” labels, are not included.   Proper wash water treatment before disposal is essential.

Best Practices Summary

Design

·         Match the washing step to the grinding step.  Some wet grinders generate enough heat to raise water temperature without the need for extra water heating.  Some manufacturers sell grinding and washing systems that have been optimized to operate under conditions that may be different from other systems.  The grinding and washing designs should be coordinated and compatible.

·         Size the washing tank to the expected throughput of material.  Plan on a 4:1 ratio of wash water to HDPE flake.  Plan on a 30 minute wash cycle followed by a 10 minute skimming step.

·         Baffled upright cylindrical tanks are great for washing.  Avoid tanks that have “deadcorners” where material settles and compacts.

·         Provide satisfactory means for removing heavy debris from the bottom of the tank.  If the bottom of the tank is dished, removal of the “heavies” is easier.

·         Install a large enough agitator impeller turning at a high enough RPM to thoroughly agitate the slurry of water and HDPE flake.  However foaming from violent agitation should be avoided.  The agitator motor should be of sufficient horsepower so as to not overheat in service.

·         Provide a water circulation and filtering system and wastewater treatment suitable for your municipal waste water system.

·         Double ground all electrical equipment and the equipment should be rated for wet service.

·         Install an eye wash station adjacent to production equipment and available to treat persons splashed with liquids.

Operation

·         Remove interfering bottles or bottles with “bleeding” labels before grinding and washing.  Interfering bottles have particularly unwelcome adhesives, labels, closures, closure liners, inserts, layers, coatings, and other attachments.  Bleeding labels release water-soluble inks into the wash water and stain the plastic flake.  The APR Design for Recyclability Guidelines indicates specific problem bottle types.  Some wash chemical systems have shown the ability to inhibit the staining, but avoiding problematic bottles is the best approach.

·         Follow the equipment manufacturer’s recommendations on wash water chemistry, water temperature, and process control.  Also, follow the manufacturers recommendations for how often to dump the wash tank contents.

·         The washing cycle should last at least 15 minutes.  30 to 40 minute wash cycles are usually best.

·         Discuss with your chemical supplier which defoamer will mitigate the foaming issue without upsetting the washing chemistry. Residue in many household product bottles may cause excessive foaming in the agitated washing tank.

·         Keep your circulation water screens clean.   Check that the flow of circulation water remains at the original copious amount and that the inlet to the circulation pump is not blocked.

·         Check the solution pH daily to determine if the proper level of cleaner is being maintained.  Exposure to air, dilution, and residues in the post-consumer bottles can alter the pH of the wash solutions.

·         When properly dumping wash water, be sure to adequately treat the water for pH, oil, suspended solids, BOD, and COD.

·         Remind appropriate employees that the wash liquids are caustic.  Eye protection should be used when working in the vicinity of the wash tanks or liquids and splashed skin should be flushed immediately with clean water.

·         Rotating equipment should not be a grabbing hazard to operators and that operators should know to keep loose sleeves away from rotating shafts and skimmers.

Maintenance

·         Position the agitator so as to not allow excessive cavitation and foaming.  At the same time, adjust the agitator speed to allow HDPE flakes to stay suspended and abrade against each other.

·         Maintain the wash chemical feed system in accordance with the manufacturer’s recommendations.

·         Eye wash stations should be operable.

Washing Post-Consumer Recycled HDPE Flake

Washing not only removes soluble contaminants from HDPE flake, but is an essential step in removal of glues, adhesives, and non-soluble contaminants.  The keys to successful washing are proper water temperature (usually hot), vigorous agitation, proper water chemistry, and proper removal of cleaned flake.

Equipment Design

The washing vessel is usually a vertical cylindrical tank with a high shear agitator.  To achieve the agitation needed, the tank diameter should be no greater than 110% of the working depth of the tank.  Baffles can be useful so that the slurry mass stays well mixed.   The agitator should be of such a design as to keep all flakes suspended, but with the impeller deep enough in the slurry that excess foaming is not created by air reaching the impeller in the slurry vortex.  The agitator should be positioned to enhance flake circulation.  Because the object in the washing step is to liberate discrete particles, dead spots of no agitation in the tank are to be avoided.  Water heating can be done on the circulating loop, drawing water out of the tank, filtering it, heating it, and returning it to the tank.  Heating coils in the tank create unwanted dead spaces.  Any steam jacketing should be insulated to conserve heat and protect operators.  The circulation loop is needed to remove filterable suspended solids, like bits of glue.   The next step is to separate particles lighter than water, such as HDPE, from particles heavier than water. This is done in a sink/float vessel with little agitation.

For a 5 ft diameter, 5 ft working height tank holding 500 lbs of flake and 2000 lbs of water, the impeller should be about 12 inches in diameter rotating at 30 rpm. The baffles should be the length of the working depth and extend about 6 inches into the tank from the wall.   A smaller impeller rotating at higher rpm will create the shearing effect needed rather than a larger, slower rotating impeller.  The impeller should be positioned off-center if the tank is unbaffled to reduce vortex formation and impaired mass mixing.  With a baffled tank the impeller can be more nearly centered in the tank.   Tilting the agitator may help keep material suspended, depending on impeller and baffle design.  The impeller should be at least as far from the bottom of the tank as the diameter of the impeller.

Depending on the scale of operation, the washing step can be conducted in a single batch operation or in a series of batch washers.  For very large-scale operations, a continuous washing process could be installed for economic improvement.  For very small-scale operations, the wash tank can be used as a settling tank by stopping agitation and circulation of water so as to allow the flakes to float.  In this case, the skimmed HDPE flake needs to be drained and rinsed so that the wash solution is not diluted with rinse water and wash solution is not lost to the sewer.

Operation

The ratio of flake to water should be about 1:4 on a weight basis.  Following the recommendation of the equipment supplier and/or the wash chemical supplier, the water typically, but not always, should be heated to 180-185 F (82-85 C).  Some recycling systems include such high shearing in the wet grinding step that wash water need not be additionally heated and some systems operate with relatively cold wash water.

For most HDPE flake washing, 1.0% caustic (NaOH) may be dissolved in the hot water before adding the HDPE flake.  Additional wash chemicals such as surfactants, chelaters, and anti-deposition chemicals may be used alone or in concert with the caustic.  Wash liquids normally have a pH of 9 to 12 and are typically considered potentially harmful to human contact.   Operators and visitors should have eye protection and splashes of wash liquid should be washed off with clean water immediately. 

For natural HDPE bottle (milk jugs, etc.) washing, the cleaning additives alone may be sufficient.  Cleaning additive systems include Union Carbide’s Triton X-100, Oakite’s RC HD, and BioSafe’s plastic bottle cleaner.  The cleaning systems are designed to separate contaminants and prevent redeposition.  Follow the manufacturer’s recommendations on concentration of additives to be used; expect to use 1 to 4% cleaner in the wash water. 

For colored HDPE, which often includes detergent bottles, additional surfactants are generally not needed, but additional defoaming additives are needed to prevent excessive foaming caused by residues in the bottles.  If using just a commercial cleaner chemical system, the operator should discuss with the supplier which defoamer to use to not interfere with the wash chemical system and how often the wash solution must be properly dumped to optimize cost and minimize any environmental effect. The appropriate federal state and local regulations should be followed.

Washing should continue for at least 15, preferably 30 to 40, minutes with agitation and circulation of liquid.  The wash tank should be so configured to allow liquid to be drawn off and pumped through a screening filter to remove glue residue and small bits of filterable waste.  The screens should be at least 100 mesh or finer.  At the end of each batch the screens should be flushed and residue disposed.  Agitation should continue throughout the washing cycle but stop at the end to allow product to be skimmed from the surface.  Continuous or series batch systems will continuously feed dirty flake and skim washed product.

Sink/Float Separation

HDPE and other polyolefins will float in the wash tank.  At the end of the typical batch washing step the agitation and circulation should be stopped and the polyolefin flake should be allowed to float and be skimmed off to a rinsing step.  Typically, skimmed flake is allowed to drain back into the wash tank to preserve the quantity and integrity of the wash solution.  Some sinking of heavier pieces, such as PET, glass, and metals, will occur now in the wash tank.  The skimmed and drained polyolefin flakes are rinsed and passed to a sink/float step in a still water tank for removal of any remaining heavy contaminants.  The still water tank may have very slow agitation so heavy particles have the opportunity to sink and polyolefin particles have the opportunity to float. Be aware that polypropylene, low density polyethylene, and various types of high density polyethylene will float together.  The “heavies” are removed from the still tank and wash tank by siphon or underwater auger, dewatered, and disposed.  The floating pieces are skimmed and dried of most surface water in spinning dryers.  Hydrocyclones may also be used to separate particles with specific gravities greater than 1.0 from the polyolefins, which have specific gravities less than 1.0.  The hydrocyclone uses hardware design and the energy of flowing liquids to magnify the gravitational settling rate seen in the still water tank.  Hydrocyclones can be configured to separate some polyolefins from other polyolefins, all having specific gravities less than 1.0.  But hydrocyclones cost more than sink/float tanks to install and operate and are very sensitive to particle size and shape.  Whether after sink/float or hydrocycloning, the washed and rinsed flakes are dewatered in spinning dryers to levels of water below about 1 to 2%.  Depending on the bottles being processed, air elutriation may be conducted at this point to remove unwanted label material.  This has the added benefit of increasing the bulk density of the flake, which improves the efficiency of subsequent operations, especially extrusion. The finished washed flake would then be ready for storage, shipment, or melt filtration and extrusion.

Because the washing and sink/float steps do not separate the various polyolefin resins from each other, the operator should control the bottles being processed so they do not contain resins or colored parts that create a degradation of quality in the final product.  Depending on the end use, varying amounts of LDPE, LLDPE, and polypropylene of various molecular makeup can be tolerated.  Some thin plastics with specific gravities greater than 1.0, such as PVC used for labels and nylons used as barrier layers, may float due to adhering air bubbles.  The gentle agitation in the sink/float step should be adjusted so the non-polyolefins can sink.  Even so, some films may stay with the HDPE flake and need the post-washing air elutriation.

Waste Water Consideration

Maintain a complete understanding of federal state and local municipal permitted limits for the quantity and nature of wastewater discharged to municipal sewers.  Disposed water should be skimmed to remove oil and settled to remove sinkable solids.  Filtration may be required to remove suspended solids to levels allowed by the local regulations.  Wash water should be neutralized before discharging to an approved wastewater treatment system, typically done to pH below 9 by the addition of sulfuric acid.  The BOD (biological oxygen demand) and COD (chemical oxygen demand) should be below maximum limits allowed by federal state and local authorities.  The temperature of the discharged water should be acceptable to the municipal sewer authorities.  Confirm allowable limits on all wastewater discharges and have satisfactory records available to demonstrate compliance.