Page 208 - Materials Chemistry, Second Edition

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Sustainability of Municipal Solid Waste Management
Changing the pulping technique has effects on the efficiency of pulp
separation as well as effects on the properties of the paper produced. During
beating, it was observed that as the carton pieces continuously pass between
the rotating drum and the bed plate, small fine pieces of polyethylene and
Al/PE layers are cut. These fine pieces were able to pass through the screen
slots during the screening process and get mixed with the separated pulp fibers.
Much fewer Al and PE impurities were present when the hydrapulper was
used. At the same time, the rate of separating the pulp fibers using a beater
was higher than that using a hydrapulper. As for the effect of changing the pulp-
ing technique on the paper properties the tensile, bursting, and tear strengths
and stiffness of paper produced using the beater are compared to those produced
using the hydrapulper. It can be concluded that the tensile and bursting
strengths of paper are higher, while the tear strength and stiffness of paper
are lower when the beater is used compared to the hydrapulper. So, depending
on the paper product and its required paper properties the most suitable
technique among the two studied techniques should be used. Tensile strength
is necessary during printing or converting paper into useful products where
the resistance to web breaking is important. Bursting strength is important
for paper bags to withstand any applied pressures and avoid rupture. Tearing
resistance is important for different products such as newsprint, books, and
magazines as well as paper bags and wrapping paper. Stiffness is important
for box boards and corrugating medium and a certain degree of stiffness is
required for printing papers.

5.11 Recycling of Laminated Plastics
Laminated plastics are polyethylene coated with a very thin aluminum layer
and are used in chip bags, for example, and what is left from milk/juice car-
tons after fiber removal as discussed above. Charlier and Sjoberg (1995) con-
sidered the separation of the aluminum foil and the polyethylene film a
critical task. They presented two different techniques for handling residues
from the repulping facilities based on a technical feasibility study done by a
researcher and a leading beverage carton producer. Both techniques aim at
material recycling of aluminum and energy recovery of polyethylene start-
ing with the removal of the undesired fibers and objects in the remaining
residue. This can be achieved by first shredding the residue using monoshear
type equipment followed by magnetic and eddy current separation. An
aluminum-rich fraction representing about 50% in weight of the incoming
material is obtained. The polyethylene film in the fraction is then thermally
removed using one of two alternative techniques. One technique is thermal
decoating based on a rotary kiln for decoating, where the incoming material
is efficiently stirred inside the rotating kiln drum and directly heated by hot
gases or by contact with the heated kiln wall. The other technique is ther-
mal decoating based on a moving bed pyrolysis oven, where the incoming