Wednesday, June 20, 2012

What Are the Different Machines Used in Recycling?

Recycling of household waste requires a combination of machines to separate it into purified products which can be used as "new" raw materials (i.e. recycled materials).

The easiest sorting is done by hand, and by mechanical sorters which depend upon physical properties. Examples are to remove ferrous metals with a magnet, or to sort plastic containers from glass based on weight characteristics. Paper is often separated out due to its being lightweight and flat. Machines to accomplish these separations include augers, trammels, air sorters and paddles of various shapes.

However the future of more sophisticated separation techniques is really in the area of optical sorters. These rely on a scanner/camera which analyzes the items being scanned for various characteristics. Optical sorters include near infrared (NIR) and x-ray scanners. All have been in use in Europe for over twenty years, but have become common in the United States only in the past few years.

Cameras use the reflective quality of light to determine the color of objects. This is most often used to separate glass by color. These can be finely tuned to detect even lightly tinted bottles.

X-ray scanners are most often used to detect PVC plastics (#3) and separate them from other plastic materials.

By far, the most versatile of the optical sorters are the NIR scanners. This technology sends signals in the shorter portion of the light spectrum. Some of these are absorbed by the object being scanned. Based on the light which is transmitted back, the chemical characteristics of the object can be determined. This type of scanning can separate plastic types #1,2 and 4-7, which includes PET, HDPE, LDPE, polypropylene, polystyrene, and also aseptic layered beverage containers (milk and juice cartons) and metals such as copper.

NIR scanning ignores labels which may remain on containers, resulting in a high degree of accuracy.

To complete the sorting, the scanned item is knocked off the conveyor into a bin, usually by a directed jet of high-pressure air. This is coordinated with the item's placement on the belt.

Systems may be mass or singulated feed. Singulated feeds send one item at a time past the scanners, which increases accuracy, but also slows the process and causes more jamming and breakdowns.

The success of resource recovery from recycling operations depends on fast and accurate sorting. Optical scanners are an important part of this process, and the technology continues to improve.