Problems With Heteroatoms

The major hetero-atoms appearing in polymers are: oxygen, nitrogen, chlorine, bromine, fluorine. After plastics pyrolysis, these elements either appear as intermediate organic compounds still incorporating the hetero-element, or as stable inorganic compounds, i.e. water, ammonia and hydrogen cyanide, hydrogen chloride, hydrogen bromide and bromine, hydrogen fluoride. Most of these are hazardous and corrosive and require a careful selection of construction materials, as well as methods to neutralize or inhibit their effect.

The presence of halogenated polymers and fire retardants, of heavy metals, and the potential formation of dioxins are some of the problems addressed in various studies presented at the successive Symposia on Feedstock Recycling (ISFR). Dehalogenation is a major topic. Some important commercial polymers (PVC, PVDC, chlorinated PE) introduce the element chlorine in almost any mixed feedstock, including those that are derived from MSW plastics, WEEE, or ASR materials. Moreover, the latter two streams also contain brominated fire retardants that may pose problems during recycling. The following important topics were addressed at the 2nd ISFR Symposium:

  • elimination of chlorine from mixed plastic fractions, to produce oil, coke and gas fractions, free from chlorine;
  • scavenging of halogens from a reaction mixture;
  • scrubbing of HCl and HBr from an off-gas flow;
  • closing the Br loop, to create a sustainable solution to the problem of fire retardants;
  • interactions between the flame retardants based on Br and on Sb in feedstock recycling.

Dehalogenation has been studied at several levels, namely that of (Contributions presented at the 2nd ISFR Symposium [9]):

  1. Dehalogenation prior to or during the pyrolysis process. Okuwaki and Yoshioka monitored debromination of Printed Circuit Boards (PCBs), at rising temperatures while heating these under helium. These researchers treated various products arising from plastics liquefaction plant in an autoclave using a mixture of NaOH and water, and obtaining deep dechlorination after 3 h at 250°C. H. Ishihara and M. Kayaba (Hitachi Chemical Co. Ltd., Japan, Sony Co., Japan) focused on epoxy resins, used in most PCBs, and developed a technology to depolymerize brominated epoxy resins in a solvent, in the presence of alkali metal compound. Thus PCBs separated into resin solvent, glass cloth, and metals, including mounted devices and solder. M. P. Luda et al. studied the thermal degradation of brominated bisphenol A derivatives. Sakata et al. used iron oxide and calcium carbonate compounds to produce a halogen-free oil, or treat gaseous effluents. Kamo et al. studied liquid phase cracking in H-donor solvents, such as tetralin and decalin.
  2. Dehalogenation of liquid products, using a catalyst prepared from goethite and phenol resins for the dechlorination of chlorocyclohexane as a test substance (Matsui et al.).
  3. HCl and HBr removal from gaseous effluents. Bhaskar et al. examined Ca-, Fe-, Zn-and Mg-based sorbents for this purpose, obtaining good results with Ca-Z. Hakata et al. tested an iron oxide-carbon composite catalyst for the selective vapour phase dechlorination of chloroalkanes.
  4. Dehalogenation in the presence of antimony, studied by Uddin et al.
  5. Closing the bromine cycle, with initiatives of the European Brominated Flame Retardant Industry Panel (EBFRIP) in cooperation with the Bromine Science and Environmental Forum (BSEF), including a study prepared by ECN (Petten) on a two-stage pyrolysis/high-temperature gasification process.

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    How to neutralize bromine and chlorine in pyrolysis of plastics?
    9 years ago

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