Three types of screens are used in the processing of refuse:
trammel screens, flatbed screens, and disc screens. The first two types of
screens employ rotating and vibrating actions, respectively, to cause material
to repeatedly fall onto the screening surface (composed of wire mesh or
perforated plate), and to progress along the screen. In the case of disc
screens, a series of rotating discs move the material horizontally along the
screen surface. Material falling through a disc screen is governed by the
dimensions of the spaces between the discs and between the shafts on which the
discs are mounted.
Screen openings generally are either round or square. The size of
the opening varies from 0.5 inches (13 millimeters) to 4 inches (102
millimeters), depending on the location of the units within the overall
processing plant.
Screening is typically used in solid waste processing systems to
improve the quality of the stream fraction destined for use as a refuse-derived
fuel. Quality improvement is accomplished by removing the finely sized
particles. The finely sized particles are predominately high in ash content
and in moisture content, and low in heating value. The average particle size
of RDF can range from 1 inch (25 millimeters) to 4 inches (102 millimeters).
Screens can be used for several other purposes in solid waste
processing such as scalping large pieces of cardboard from the raw solid waste
stream, concentrating aluminum cans, and controlling the size of particles
destined for incineration. The concentration of cardboard and cans is
generally done with "pre-trommel screens," that is, screens employed prior to
size reduction of raw solid wastes.
6.1.5
Magnetic Separation. Magnetic separation is a process whereby
ferromagnetic materials are removed from a mixture of magnetic and nonmagnetic
materials through the application of a magnetic field. Three types of magnetic
separators are used in refuse processing systems, namely the overhead belt
magnet, the magnetic head pulley, and the drum magnet. Each type of separator
relies upon magnetic attraction and ballistics to effect the recovery of the
magnetic ferrous materials. The purity of the recovered ferromagnetic material
is a function of the extent of upstream processing, of the throughput, and of
the strength and spatial arrangement of the magnetic separation system.
Densification. As it applies to refuse-derived fuel, densification
6.1.6
consists of the formation of high density unit particles through the
application of high pressure to a confined mass of properly prepared
combustible fraction of solid waste. The unit particles can take the form of
pellets, cubes, or briquettes and typically have linear dimensions on the order
of 0.8 to 1.6 inches (20 to 40 millimeters). The rotary die pellet mill is the
most commonly used device for the densification of refuse-derived fuel. In a
typical pellet mill application, air-classified light fraction (or screened
air-classified light fraction) is forced against the inner face of a
cylindrical die whereby the material is extruded through radially oriented
holes in the die. The feedstock is compressed as a result of friction between
the material and the walls of the cylindrical die openings. Densified
refuse-derived-fuel (termed "d-RDF") has a bulk density that is about 5 to 10
times that of the RDF feedstock, and is thus more easily transported and
stored than fluff (i.e., loose) RDF. Also, combustion of d-RDF is the
preferable form of RDF for some types of boilers.
5.10-44
Previous Page
