Subcritical Hydrothermal Extraction of Chloride and Inorganic Additives From PVC Waste

Abstract

Waste polyvinyl chloride (PVC) poses recycling issues. Often waste plastics serve as an energy source in the form of refuse derived fuel (RDF), which is applied in, for instance, cement production or energy (heat and electricity) through incineration. However, the addition of waste PVC to such RDF is limited due to the production of corrosive chlorine gas during incineration. Furthermore, commercial plastics are materials composed of the main polymer with organic and inorganic additives that enhance the material properties (e.g. elasticity, durability, fire resistance) [1]. A commonly used fire retardant material is antimony trioxide (Sb2O3). In the USA, the major end-use of antimony (35%) is as fire retardant material [2]. Antimony is a valuable metal and is considered to be a critical raw material in Europe due to its supply risk and economic importance [3]. Furthermore, modern-day plastic recycling has to deal with legacy additives, which were allowed when the plastic material was produced but are now restricted or prohibited through regulations, such as Cd [4]. Currently, removing additives from plastic waste is technically and economical difficult. It has been shown that hydrothermal carbonization (HTC) converts pure PVC to polyene and the released chloride ions are dissolved in the water phase [5]. However, this process has not been further explored to treat commercial PVC waste samples containing (inorganic) additives.

In order to avoid the loss of the commodities antimony and polymeric carbon, and the emission of toxic and corrosive chlorine gas to the atmosphere and heavy metals to ashes by incineration of waste PVC, a subcritical hydrothermal process was investigated whereby PVC is converted in polyene while chloride ions and inorganic additive elements are extracted in the liquid phase. In this work, PVC from electric wire insulation was hydrothermally treated and the faith of inorganic elements, such as Cl, Sb, Ca and Zn was studied. HTC was performed at 250 °C in de-ionised water, 1 M HCl or 1 M NaOH. Parameters such as reaction time and liquid to solid ratio were optimised. The dechlorination of PVC in all three tested solvents ranges between 95 and 99% after 24 h of HTC treatment. Due to dechlorination, the pH of the aqueous solution drops during HTC treatment. After 24 h treatment the pH of the treatments in 1 M NaOH, DI water and 1 M HCl are 13.0, 1.16 and 0.60, respectively. Elements of inorganic additives (Sb, Ca, Zn) are extracted most efficiently during HTC for 24 h in acidic conditions (1 M HCl). Future work to optimise the resource recovery from waste PVC, such as by applying microwave heating, will be outlined.

References
[1] R. Babinsky, Plastics Additives & Compounding 2006, DOI: 10.1016/S1464-391X(06)70526-8, 38-40.
[2] R. R. Seal II, K. J. Schulz, J. H. DeYoung, D. M. Sutphin, L. J. Drew, J. F. Carlin and B. R. Berger, Antimony, 2017.
[3] Deloitte Sustainability, British Geological Survey, Bureau de Recherches Géologiques et Minières and Netherlands Organisation for Applied Scientific Research, Study on the review of the list of Critical Raw Materials, European commission - DG GROW, 2017.
[4] A. Sevenster, J Mater Cycles Waste Manag, 2012, 14, 281–285.
[5] J. Poerschmann, B. Weiner, S. Woszidlo, R. Koehler and F. D. Kopinke, Chemosphere, 2015, 119, 682-689.

Keywords: PVC, dechlorination, hydrothermal carbonization, additive removal