Back-to-monomer recycling
offers a perspective for yet unrecyclable
polyethylene terephthalate (PET) waste like textile fibers, multilayer
food trays, or brittle bottles. In this context, depolymerization
by alkaline hydrolysis is a promising method which demands for consecutive
acidic or electrochemical precipitation to recover terephthalic acid
(TA). This study investigates the influence of temperatures up to
90 °C and acidification agents on precipitation of TA from aqueous
disodium terephthalate solution. The experiments were conducted with
a model reactant prepared from purified TA dissolved in sodium hydroxide.
The influence on the TA crystal size and morphology affecting further
processing, such as filterability and flowability, are discussed.
In comparison to commonly used bulk chemical sulfuric acid, experiments
with acetic acid were conducted. An enhanced solubility by diluted
acetic acid and elevated precipitation temperature lead to significantly
larger crystals. Likewise, filtration times for sulfuric acid can
be shortened by more than 80% upon increasing the precipitation temperature
from 36.5 to 90 °C. The comparison of yield in dependence of
pH for different precipitation acids (phosphoric, hydrochloric, oxalic,
citric) emphasizes the effect of solid product removal since even
weaker acids than TA lead to a reasonable yield of up to 78% for acetic
acid. These results offer a perspective and show the necessity for
optimizing precipitation with respect to product quality and processability.
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