NaOH–KOH Capillary Action Enhances the Pyrolysis and Debromination of Waste Printed Circuit Boards

Abstract

Catalytic pyrolysis was one of the most promising technologies for energy recovery and bromide removal from nonmetals of waste printed circuit boards (NM-WPCBs). However, the low efficiency of debromination, large dosage of catalysts, and high consumption of energy were the main reasons for limiting the application and popularity of catalytic pyrolysis. To solve these problems, the capillary action of molten NaOH–KOH was used to enhance the pyrolysis and debromination of NM-WPCBs. In this paper, the molten state of NaOH–KOH was achieved at an NaOH/(NaOH–KOH) mole ratio of 0.5 and a temperature of 182 °C. The maximum debromination efficiency reached 95.0%, while the average activation energy decreased from 201.4 to 112.6 KJ/mol. Kinetic results suggested that nucleation and diffusion action played key roles when α was below 0.6. Herein, the enhancing mechanism for the capillary action of catalysts was analyzed. First, molten NaOH–KOH spontaneously spread out on the surface of NM-WPCBs with the capillary force. Then, the Br atoms coordinated with Na/K to form [NM-WPCBs-Br···NaOH/KOH], in which the electrons transfer from Br to Na/K, resulting in the cleavage of the C–Br bond. Afterwards, the bromine radical (Br*) bonded with the hydroxyl radical (H*) to form HBr, which was removed by NaOH–KOH. This work offers inspiration for recycling other plastics by removing contamination and improving the efficiency.

Recent Advancements in Pyrolysis of Halogen-Containing Plastics for Resource Recovery and Halogen Upcycling: A State-of-the-Art Review

“…Recently, a variety of catalysts have been investigated for the in situ catalytic pyrolysis of halogen-plastics, as summarized in Table S3. As the alkali and alkaline earth metals, the strong alkalinity of Na- and K-additives limits their applications in pyrolysis, although high debromination efficiencies are achieved by the molten salts process. , Notably, Ca-additives demonstrate remarkable debromination efficiency for halogen-plastics. − It indicates that the dehydration of Ca­(OH) 2 that occurred to CaO is effective in removing Br due to charge transfer, resulting in over 96% debromination efficiency . Transition metals and metal oxides, characterized by cost-effectiveness, minimal corrosion, and excellent catalytic effects, have also been investigated.…”

Debromination with Bromine Recovery from Pyrolysis of Waste Printed Circuit Boards Offers Economic and Environmental Benefits

“…Several studies have been conducted on copyrolysis debromination, and these methods can improve the quality of pyrolysis oil and gas compared to traditional pyrolysis. − However, insufficient contact between catalyst and reactants, deposition of pyrolytic carbon on the catalyst surface, and adsorption of bromides on the catalyst surface resulted in low efficiency of copyrolysis debromination. , So these pyrolysis products cannot be directly used as industrial feedstock or fuel . Therefore, to improve the quality of pyrolysis products, reduce secondary pollution, and reutilize bromine resources, the following research directions are envisaged: 1) precise synergistic pyrolysis (rematching of the metals and different elements contained in the circuit boards and the corresponding accelerated elemental reorganization) and deep debromination need to be further studied, 2) combining in situ precatalysis and ex situ catalysis to improve catalytic debromination efficiency, 3) using multiatmosphere pyrolysis (such as H 2 O, CO 2 , CH 4 ) to regulate the migration and transition of bromide in different phases, meanwhile accelerating the rematching between different elements, and 4) studying the physical adsorption of adsorbent under low-temperature pyrolysis conditions for debromination, bromine fixation, and energy savings. , …”