All plastics, irrespective of their prior use or application are processed and are size reduced and sanitized within the autoclave. They emerge from the autoclave and are separated from the treated mass through the use of a rotary screen or trommel. The plastics are diverted onto a separate moving table over which is situated a number of robotic laser operated plastics segregators.
The sensors are pre-programmed with the density, specific gravity and color characteristics of each type of plastics. There are in essence seven different types of plastics. These are:
- PET (polyethylene terephthalate) e.g. 2 liter soft drinks bottle
- HDPE (high density polyethylene) e.g. containers for liquids e.g. milk, household products
- V (vinyl) or PVCs — Polyvinyl chloride requires special attention due to its high content of dangerous chemicals, e.g. phthalates (to make toys soft and flexible), lead, cadmium and organotin compounds. The high chlorine content poses a risk of dioxin when incinerated, particularly in uncontrolled burning. In certain conditions phthalates can leak out of the plastic, e.g. when chewed by children
- LDPE (low density polyethylene) e.g. plastic bags
- PP (polypropylene) e.g. ropes, bottle crates and car battery boxes
- PS (polystyrene) e.g. packing, insulation, disposable cups, fast food boxes
- Other (including multi-layer) PET, HDPE, LDPE and PVC. All these form the bulk of the plastic packaging recovered
The sophisticated sorting technology makes it possible to fully automate separation of recyclable materials. The input stream is analyzed by a fast scanning sensor installed over a conveyor belt. It rapidly identifies materials, shapes, textures and colors as well as the object position. The sorter blows the defined sorting fraction onto a second transport system while the residual fraction is brought to a third belt for further sorting or disposal. The system has superior performance and exploits all the possibilities of economic separation of recyclable materials.
- Capacity ranging from one to ten tons per hour
- Hit-rates around 90%, but in some cases even higher. This is the percentage of the targeted material in the input stream that is ejected
- Purity rates typically between 90 and 93%, but could reach 98% in some applications. This is cleanliness of the ejected material (in percentage)
- The units can sort waste streams from different sources, regions and seasons.
- The sorting criteria can be easily adjusted with just one click.
- Software upgrades can be downloaded via a modem
- Negligible mechanical wear.
- Stringent tests and approval procedures before starting-up at a customer location.
- Reliable components successfully used in many applications.
- Effective remote diagnosis and maintenance through a modem.
- Service engineers worldwide to secure down-time to a minimum.
- Preventive service agreements available.
This very sophisticated sorting equipment (for which there are 740 global and US reference sites) identify the plastics, and using air powered paddles flip the different plastics into the respective silo. The separated material within a silo is periodically passed through the same machine down line. This re-analyzes the material to remove other missed or foreign matter. It then separates by color. It is designed to specifically sort polymers or mixed plastics by material properties. With its NIR (near infrared) sensor, it efficiently detects the characteristic infrared wavelength of light reflected by illuminated objects. Thus you obtain a fingerprint of light – unique for every different type of material.
Post sanitization within the autoclave the plastics are segregated into the seven different poly types of plastics. These are then shredded and rewashed and passed through a proprietary plastics recovery plant where, using thermal energy, the plastics are melted to a viscous state and passed through bubbling liquids to produce FSDA approved food grade plastic granules on site as part of the recovery process. These granules can be bagged in 50kg bags for off-site sales to third party plastics converters or used immediately and converted into other plastic products in an adjacent building. This ensures that the value within the plastics stream is retained within the plant and also reduces handling and freight costs of baled plastics to third party converters. Revenue from this activity offsets the operating cost of the whole STAG treatment plant.
Plastics to Diesel
The thermal de-polymerization process can convert a wide range of waste materials into oil and other useful by-products, in proportions that vary according to the specific type of feedstock run through the works.
Using proprietary de-polymerization process the small fractions of waste plastics not capable of being sorted (such as heavily printed thin film bags and containers) can be sanitized and converted back to premium grade diesel fuel leaving minimal residue and a conversion rate of just over 86% for every ton of plastics feedstock to produce 1 ton of diesel fuel. Plastics that are difficult, such as vinyl, polystyrene and polyethylene are not readily recyclable as plastic granulate. They do however lend themselves to being deconstructed through a globally acceptable and widely licensed depolymerization process that reverts the plastic back to its original feedstock – i.e., diesel fuel. It should be perhaps remembered that it requires 10,500 barrels of oil to produce 1000 tons of plastic. So reverting it back to oil is an accepted process licensed already in the United States and it is this process that will be used to produce value from what is accepted as being plastics that cannot be recycled.