The depressing effect of CaLS on plastic is attributed
mainly to its adsorption on the surface of the plastic. Lignosulfonate
possesses a hydrophobic non-polar hydrocarbon
group and a hydrophilic polar sulfonate group. The
molecules adsorbed on the plastic surface expose some of
their polar groups oriented toward the aqueous phase,
hence making the plastic surface more hydrophilic (Fraunholcz,
2004) and causing higher wettability. This result
reflects its contact angle (see Figs. 3 and 4). When plastic
becomes more hydrophilic, its contact angles for those flotation
agents are decreased. Upon adding CaLS to
500 mg l1, the contact angle drops from 80 to 73 for
PET, 87 to 79 for PVC, 88 to 76 for PS, and 89 to
70 for ABS. In order to be successful in selective flotation
separation, an appropriate dosage of reagents is chosen to
render the surface of one plastic typology more hydrophilic
than other plastic typologies. Differences in contact angles,
and thus differences in wettability, result in selective flotation
behavior. Therefore, on the basis of the abovereported
results, CaLS with a concentration of 500 mg l1
and CaLS with a concentration of 200 mg l1 were chosen
for further PET/PVC separation and for further PS/ABS
separation studies, respectively.