What Works?

GOALS AND STANDARDS. In principle, all non-paper components should be removed as far as possible from the product stream at the time of recovered paper sorting. The criteria for determining the percentage of "unusable materials" for grades defined in the European paper industry’s EN 643 document will be subject to individual mills’ specifications. In the case of a high concentration of unusable materials (e.g., more than 3 percent), customers most often refuse delivery of the recovered paper shipment.

In Germany, it is mainly graphic paper for de-inking that is sorted. Among the detrimental substances that must be removed by sorting to achieve the specified quality are boards, unwanted papers and non-paper materials. In practice, sorting is often done in the form of manual negative sorting, i.e. all detrimental or unwanted components are removed from the material stream.

Besides the sorted graphic paper for de-inking, the recovered paper trade also offers unsorted qualities as well as (sorted) mixed papers and boards. The decisive factor for sorting process profitability is the difference in revenue between the sorted grades, (supermarket corrugated paper and board) and the mixed qualities. As the efficiency of recovered paper sorting depends on market conditions—supply and demand for various recovered paper grades—the volume and quality of the recovered paper is subject to considerable fluctuations.

Paper producers need a high and constant recovered paper quality as well as sufficient volume. These demands lead in part to significantly increased numbers of complaints as well as a greater demand for recovered paper grades with a controlled quality.

AUTOMATION OPPORTUNITIES. Mechanical processing of recyclables in Germany has a history going back more than 30 years. The applied technologies were modeled on processing techniques used in mineral technology and agricultural industries. The introduction of Germany’s Dual System, which was put in place in the early 1990s to meet the goals of packaging recycling legislation, led to technical innovations.

The first steps toward automating recovered paper sorting were undertaken beginning in about 1987. Since then, mechanical sorting techniques have been used to support the manual sorting process. With these mechanical processes, non-de-inkable components, such as OCC, can be separated.

Since about 2002, optical sensors and image processing methods have been used in recovered paper sorting. Despite a large number of different separating combinations used in practice, separation is always realized on the basis of a few stock properties or physical effects. Those properties include size, stiffness, color, composition and weight. The methods use to separate based on those qualities include:

SCREEN TESTS. Screen classification serves especially to accumulate or deplete certain stock groups in the separated stock streams.

The efficiency of a screen depends on the quality of spreading out the material. If material remains layered, many grains cannot pass through the screen, even though their dimensions would allow it otherwise. For example, a large cardboard box at the bottom of a pile of recovered paper may prevent adequately sized material from falling through the screen.

A number of instruments are used in practice to separate large cardboard products. For instance, ballistic separators are screens that support material transport and screening by rotating. The rotating movement can be activated, for example, by driving the screen via an off-center crankshaft.

Disc screens consist of lines of several driven shafts on which discs sit at a defined interval. Depending on the separation task, the size of the discs varies, as does the distance between the shafts. The material is conveyed by the rotation of the shafts fitted with the discs.

Star screens are theoretically comparable to disc screens, but, instead of discs, star-shaped sorting elements are arranged on the rotating shafts.

Drum screens are cylindrical bodies with screen apertures around the circumference. Depending on the purpose for which they are intended, the screen apertures have different sizes and geometries. The defined angle of the screen drums combined with the turning motion conveys the material. Fittings in the screen drum can support the material mixing in an aimed way or slow the throughput. Both the geometry and the size of the screen apertures can vary along the length of a drum body. Drum screens are usually driven via the circumference.

The gap technique does not describe a screen in the conventional sense, despite the fact that separation is achieved by the defined geometry of the separation device. Stock stream is separated by means of adjustable gaps between the conveyors, whose speed and intermediate spaces can be set. The gap can be adjusted horizontally and vertically to achieve different effects.

The above techniques offer the following advantages for separating large-sized cardboard products: proven, reliable technique; minimal downtime because of maintenance; installation is usually possible without major reorganization measures; and insensitivity to non-paper components.

A disadvantage can be that crushed or folded cardboard is not ejected.

COMBINATION PLATTERS. The following processes have shown efficiency in practice as additional possibilities to automate recovered paper sorting:

Care must be taken here to ensure that a coarse screen always precedes all three steps.

A fine screen after the coarse screen separates small papers or other components that succeeded in passing through the screen apertures from the paper stream. The fine sorting is usually at about 100 millimeters. This considerably further facilitates a following sorting of the paper stream, as the separation of small elements is highly labor intensive. In principle, the same instruments are used for fine sorting as for coarse sorting, the difference lies in the size of the screen apertures or gap.

Before a "paper spike" or a "de-inking screen" is used, the recovered paper stream should be pre-treated by coarse and fine sorting. Both large and small components should have been extensively removed beforehand. In the "paper spike" or comparable systems, the recovered paper mixture that is to be sorted runs on a conveyor underneath a row of parallel V-belts that are equipped with projecting nails, or "spikes."

The spacing between these V-belts and the conveyor can be adjusted. Stronger materials, such as cardboard and boxboard, are pierced by the spikes arranged in the V-belts. Less-firm materials bend under the pressure of the spikes and are not pierced. After passing the gap, the recovered paper that is not spiked by the nails directly falls on to a removal belt.

Stronger cardboard and boxboard that is fixed by the spikes is directed to the end of the installation where it is removed from the spikes by a counter-rotating roller and ejected separately from the installation. The "de-inking screen" manufactured by Bulk Handling Systems (BHS), Eugene, Ore., combines the disc sorting technique with a vacuum system. With the aid of a vacuum, the perforated rollers of the screen suck up all recovered paper components, causing flexible components to be separated from boxboard material.

For years, optical recognition units have been used successfully to sort plastics as well as glass. This technique is being used increasingly for recovered paper sorting.

To achieve a successful sort, the recovered paper stream on the feed conveyor must be presented in a single layer. Fine sorting for separating small recovered paper components is also important, as it considerably simplifies the job of the recognition units.

In addition to the sorting technique, an air separator can be positioned before the recognition systems. In the air separator, the material to be separated is directed in its original size by a speed-adjustable acceleration conveyor into the separating space.

OPTICAL OPTIONS. The optical recognition system for sorting graphic paper for de-inking from mixed qualities can consist of the following components:

The images recorded with a high-resolution camera are evaluated using image processing and pattern recognition. The most important property here is the color of the detected object. With the aid of a color camera, it is possible to recognize brown and gray boxboard as well as papers that have been dyed throughout. According to the manufacturers, distinguishing between gray newspapers and gray boxboard is a problem. Mistakes can be made also with color. For example, a brown area in a magazine may mistakenly be identified as brown boxboard. It is not possible to distinguish printed boxboard products from color magazines and safely eject them.

The CMYK sensor is able to recognize whether an object was printed with three or four colors. CMYK stands for the colors of cyan, magenta, yellow and black, which are commonly used in color printing. Three-color printing manages without black.

In view of the fact that printing on boxboard does not usually require an extremely high-quality printed image, it is done mostly using the three-color printing process. Whereas the high-resolution color camera can possibly mistake brown boxboard for magazines with brown print, safe detection with a CMYK sensor is considerably more likely, as brown board is seldom printed using the costly four-color process.

Similarly, the recognition of colored papers is possible because they are never printed using the four-color process, whereas identical colors in magazines are printed using the four-color process.

The combination of CMYK sensor and high-resolution camera offers a higher degree of recognition safety because of redundancy. Both systems can recognize color and mutually support one another in the evaluation. Therefore, the combination of these sensors offers a relatively accurate recognition of boxboard and dyed-throughout papers. It also allows the ejection of certain printed boxboard.

NIR sensors detect the adsorption in the infrared wavelength range. They are capable of recognizing the entire spectrum of materials from domestic waste collections (DSD), such as plastics and beverage cartons and sheer layers, which are commonly used on color packaging for frozen food products, when the sensor has been calibrated accordingly. Therefore, a NIR sensor is used in recovered paper sorting to recognize "foreign" materials and their compounds with boxboard.

The advantages of using optical recognition systems for recovered paper sorting are:

Drawbacks include:

RISKS AND REWARDS. Besides the screening techniques that have been in use for a relatively long time, other purely mechanical processes are being applied increasingly to separate recovered paper mixtures. Similarly, sensor-assisted separating processes are in increasingly widespread use for sorting.