Precipitation of Terephthalic Acid from Alkaline Solution: Influence of Temperature and Precipitation Acid

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.