Paper Separation, Paper Sensing & Paper Sorting
Paper separation from other recyclables most efficiently is accomplished by the user, before the paper enters the waste stream. Once paper is mixed with other recyclables, removing and sorting it becomes tricky. That said, consumers rarely do a careful job of sorting, and many facilities are now accepting completely mixed waste. This reduces collection costs and eliminates the frustration of unmet expectations concerning correctly pre-sorted materials.
At the simplest (other than human sorting), the mixed waste passes over a rolling drum with paddles or cams. Because the paper is lighter than other containers, and flat, it is propelled forward and the heavier, more dimensional materials fall out behind the drum.
Once past this point, the sorting of paper grades for recycling is a difficult problem. Recovered fiber can only be reprocessed into a product with shorter fiber length. Therefore, higher grade papers bring more financial return if they can be separated. Otherwise the entire batch is destined to become some low-grade product such as toilet paper.
The basic properties involved in paper sorting are lignin content, stiffness, gloss, color and adhesives content.
Lignin content can be determined by the amount of fluorescence. High lignin paper glows more brightly.
Determining stiffness, which seems simple to a human feeling a paper, is actually quite difficult for automated paper sensors. Ultrasound waves are confounded by vibration of the moving paper on a conveyor system. Piezoelectric and capacitive transducers also do not yield consistent results. A better technique has been proposed which moves single thicknesses of paper over a narrow gap in the conveyor. Air pressure is applied from above, deforming the paper downward in the gap. This curvature can be read from below, measuring the stiffness. Laser paper sensing for stiffness based on flutter as the paper is conveyed have also been tested.
Gloss is measured by means of reflectivity, and optical color paper sensing samples a sheet for an average color value. Since printed papers have varying amounts of ink coverage and background color, this allows separation of white and near-white papers (higher value) from lower value waste.
Adhesives, such as envelopes, glued boxes, and sticky notes remains a difficult problem for paper separation. A solution may be to develop adhesives that are not detrimental to the recycling process.
Paper sorting of corrugated cardboard from boxboard, such as cereal boxes, and from office paper is still best accomplished before the products enter the automated materials recovery stream.