Size
separation, or screening, involves the separation of a mixture of materials
into two or more portions by means of one or more screening surfaces, which are
used as go or no-go gauges. Size separation can be accomplished dry or wet,
with the former being most common in solid waste-processing systems. Screens
have been used before and after shredding and after air classification in the
processing of refuse-derived fuel (RDF). They are also used in the processing
od compost and mulch to produce a more uniform product. The types of screens
that are used, and the performance characteristic, design criteria, and
selection of separation equipment are discussed in this section.
Types
of equipment
The
most common types of screens used for the separation of solid waste are
vibrating screens, trommel screens (also known as rotary drum screens), and
disc screens. Each type of screen is the best adapted to specific situations.
Selection
of size separation equipment
Factors that should be considered in the selection of size screening
equipment are summarized below.
-
Waste characteristics: particle size, shape, bulk specific weight,
moisture content, particle size distribution, clumping tendency etc.
-
Materials specifications for screened
components: performance
characteristics of screen should match required product.
-
Screen design parameters: Size of opening (in), percentage open space,
total surface screening area(ft²),
oscillation rate for reciprocating screens (times/min), rotational speed for
trommels etc.
-
Separation efficiency: recovery (%), efficiency (%), purity (%).
-
Operational characteristics: energy requirements (hp), maintenance,
complexity of operation, noise, and air and wastewater emissions.
-
Site factors: floor space and vertical space availability, access.
Because
screens are available from a number of manufactures, designers should develop
performance-based specifications for recovery, effectiveness, and purity to
choose the most cost-effective screening system to meet their needs. Designs
made with the methods suggested in this section, from the literature and from
manufacturers’ data, should be verified by measurements made at operating
plants or by pilot testing.