The fate of bromine during microwave-assisted pyrolysis of waste printed circuit boards

Abstract

Bromine control is imperative for efficient treatment and products utilization during pyrolysis of waste printed circuit boards (WPCBs). This study investigated Br-species in products from microwave-assisted auger pyrolysis of WPCBs, and discussed synergetic evolution mechanisms, release kinetics and thermodynamics of Br-containing pollutants with different kinds of mineral species (alkaline earth, alkali, and transition metals). Results indicated that heavy Br-containing volatiles release (e.g., brominated phenols) was dominated at 320–520 °C. Brominated phenols released Br* to react with small-molecule groups to form light Br-containing products (e.g., HBr, CH3Br, and CH3CH2Br) at >520 °C. K2CO3 efficiently suppressed Br-containing pollutants emissions (∼50% reduction) and promoted bromine fixation in char (∼33.49% increase). With K2CO3 addition, bromine evolution mechanism is largely dehydrobromination and neutralization reactions when bromine bonds with aliphatic carbon with an adjacent aliphatic hydrogen. Negatively charged oxygen of K2CO3 attacks bromine and causes C-Br scission when bromine bonds with CH3* or aromatic carbon. The chemical reaction models (CRM3–CRM5) are best fitted with bromine evolution and the activation energy of WPCBs-KC reached the lowest (149.83–192.19 kJ/mol). Furthermore, bromine control strategy in WPCBs pyrolysis products toward environmental and economic sustainability were suggested, which created less environmental impact and maximum resource recovery.