Material Recovery Plant

The Sorain-Cecchini MRF plant in Rome has been in operation for over twenty years. It recovers ferrous metals (6% by weight), aluminum (1%), organics (34%), and film plastics (1%) while generating densified RDF (51%) and rejecting 7% oversized items. The total plant capacity is 1200 tn per day (Cachin and Carrera 1986).

The main processing steps involve magnetic separation for ferrous metal removal, eddy-current separation for aluminum recovery, rotary screens for separation by size, and air classifiers for separation by density. The overall process consists of eighty pieces of equipment, which are flexible and can be used in different combinations as market conditions change.

Figure 10.7.2 shows the resource recovery plant in Rome. The charging conveyor 1 is provided with a pickup device 2 that breaks the bags and removes bulky reject items. A leveling device 3 meters the waste-flow rate and removes rejects. The primary screen 4 separates the large (over 8 in) fraction from the smaller, heavier fraction. The approximately 55% large fraction of paper, wood, and plastic is fed to the 10-20 rpm large breaker 5, which reduces particle size and breaks plastic bags. The large breaker automatically rejects any items it cannot break (about 2%). The output (53%) is sent to the large air classifier 6, where the lighter (10%) sheet paper and plastic fraction is separated from the heavier (43%) cardboard, wood, and rags. The reject fraction consists mostly of white goods (appliances) but includes bulky items such as bed-springs. This fraction is hauled away by subcontractors.

FIG. 10.7.2 Resource recovery plant in Rome, Italy. (Adapted from F.J. Cachin and F. Carrera, 1986, The Sorain-Cecchini system for material recovery, National Waste Processing Conference, Denver, 1986 [ASME].)

The light fraction (10%) passes through a differential shredder 7, which breaks up only the paper. It is followed by a rotary screen 8, which separates the lighter 3% of the stream, containing the plastic film. This stream is sent to the small classifier 9, where the 1% light fraction is taken to plastic recovery, while the remaining paper and rag fragments are included with the densified RDF (DRDF). The recovered plastic film (mostly polyethylene) is shredded into square-inch flakes, cleaned by washing, and air dried. The dry flakes are melted and fed to the extruder, and the pellets are shipped to plastic film manufacturers.

For metal separation, the 40% heavy fraction from the primary screen 4 passes through the primary magnetic separator 10, which removes 4% and sends that fraction to ferrous recovery. The remaining fraction is further homogenized in the small breaker 11 and separated in the secondary rotary screen 8 into the 15% large fraction (over 4 in), consisting mainly of paper, wood, and plastics, and is sent to DRDF recovery. The 21% small fraction (under 4 in), consisting of organics, glass, ashes, and aluminum, is sent to a conveyor 17; aluminum (1%) is removed by hand, and the rest (20%) is sent to organic recovery.

The 43% heavy fraction from the large air classifier 6 passes through the secondary magnetic separator 12, which removes 1% and sends that fraction to ferrous recovery. The remaining fraction travels on the sorting conveyor 13, where the semiautomatic devices and inspectors remove the cardboard (8%), which is sent DRDF. Any missed recovery items and rejects (4%) are sent to the eddy-current separator 16 for aluminum recovery and then to organic recovery. The removed aluminum is crushed and densified to a specific gravity of 1.0 (62.4 lb/cu ft) before being placed in storage. The 30% fraction remaining on the sorting conveyor 13 is mostly paper and is sent to the flail 14, where it is broken down before being sent to the secondary air classifier 15. The three magnetic separators 10, 12, and 18 send 5.8% to ferrous recovery, where it is shredded by the abrader hammer mill. The shredding step is followed by cleaning through firing or washing and a final magnetic separation step to remove the nonmetals that were loosened by the abrader.

The organics fraction, left from the plant feed after the removal of metals, plastic film, and paper, is essentially a heavy fraction of small-sized particles containing organics, glass, ceramics, sand, ashes, hard plastics, and small pieces of wood. This fraction is placed into an aerobic digester and broken down into raw compost. After the removal of glass, ceramics, and other inorganic rejects, the raw compost is subcontracted for further processing. This processing splits the organic fraction into a feed fraction (a high-quality compost fraction) and a residue, which is usually landfilled.

The 15% large fraction from the secondary rotary screen 8 and the 30% from the flail 14 are sent to the secondary air classifier 15, which removes all paper (35%) and sends it to the DRDF air classifier 19 together with the small fraction (7%) from the secondary rotary screen 8 and the heavy fraction (2%) from the small air classifier 9. After the DRDF magnetic 18 removes the remaining ferrous metals, the DRDF is densified into flakes in the recovery line. The DRDF is stored in a specific gravity of 0.6 (38 lb/ft). The heavy fraction (10%) from the secondary air classifier 15 is sent to organic recovery.

The DRDF obtained is relatively clean, and its sulfur and chlorine content is low as most metals, hard plastics (PVC, PET), and other impurities have been removed from it. The Sorain process can switch from producing DRDF to generating paper pulp depending on market condition.

Adapted from Municipal Waste Disposal in the 1990s by Bela G. Liptak (Chilton, 1991).

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