282 ENVIRONMENTAL ENGINEERINGScreensScreens separate material solely by size and do not identify the material by any other
property. Consequently, screens are most often used in materials recovery as a classi-
fication step before a materials separation process. For example, glass can be sorted
(technically but perhaps not economically) into clear and colored fractions by optical
coding. This process, however, requires that the glass be of a given size, and screens
may be used for the necessary separation.
The trommel, shown in Fig. 14-7, is the most widely used screen in materials
recoveq. The charge inside the trommel behaves in three different ways, depend-
ing on the speed of rotation. At slow speeds, the trommel material is cascading:
not being lifted but simply rolling back. At higher speed, cataracting occurs, in
which centrifugal force carries the material up to the side and then it falls back.
At even higher speeds, centrihging occurs, in which material adheres to the inside
rejectionFigure 14-7. Trommel screen.