Effects of alkali-salt fusion process on recovery of amphoteric metals from waste printed circuit boards

Algae Pyrolysis in Molten NaOH-Na2CO3 for Hydrogen Production

Biomass pyrolysis within the alkaline molten salt is attractive due to its ability to achieve high hydrogen yield under relatively mild conditions. However, poor contact between biomass, especially the biomass pellet, and hydroxide during the slow heating process, as well as low reaction temperatures, become key factors limiting the hydrogen production. To address these challenges, fast pyrolysis of the algae pellet in molten NaOH-Na₂CO₃ was conducted at 550, 650, and 750 °C. Algae were chosen as feedstock for their high photosynthetic efficiency and growth rate, and the concept of coupling molten salt with concentrated solar energy was proposed to address the issue of high energy consumption at high temperatures. At 750 °C, the pollutant gases containing Cl and S were completely removed, and the HCN removal rate reached 44.92%. During the continuous pyrolysis process, after a slight increase, the hydrogen yield remained stable at 71.48 mmol/g-algae and constituted 86.10% of the gas products, and a minimum theoretical hydrogen production efficiency of algae can reach 84.86%. Most importantly, the evolution of physicochemical properties of molten NaOH-Na₂CO₃ was revealed for the first time. Combined with the conversion characteristics of feedstock and gas products, this study provides practical guidance for large-scale application of molten salt including feedstock, operation parameters, and post-treatment process.

Catalytic effect and mechanism of in-situ metals on pyrolysis of FR4 printed circuit boards: Insights from kinetics and products

Pyrolysis is a promising method for the recovery of waste printed circuit boards (WPCBs), but few researches have noticed the influence of in-situ metals. This study conducted a series of comparisons between metal-free leftover pieces (LP) and intact boards (IB), including pyrolysis characteristics, volatile emission, kinetics, and thermodynamic parameters. The thermo-gravimetry (TG) analyses indicated that both the samples presented predominant mass loss in narrow temperature intervals, and characteristic pyrolysis temperatures of IB were approximately 15 °C lower than those of LP. Dominant constituents in evolved gases were detected by Fourier-transform infrared spectrometry as CO₂, phenol, bromophenol, ethers, ketones, and aldehydes, and metals accelerated the generation of light hydrocarbons and aromatic compounds. The activation energy and thermodynamic parameters were calculated and compared, and the results verified the presence of in-situ metals led to a lower energy barrier and higher reaction extent. Moreover, conversion behaviors of Cu, Fe, Sn, and Pb manifested the formation of metal bromides and implied the reduction of brominated volatiles. The obtained results confirmed the catalytic effect of in-situ metals on PCBs pyrolysis and their bromine fixation abilities. This study contributes to fundamental knowledge that can be used to guide the pyrolysis of WPCBs.

An environmental-friendly process for dissociating toxic substances and recovering valuable components from spent carbon cathode

Spent carbon cathode (SCC), a hazardous solid waste discharged from the aluminum electrolysis industry, has a serious environmental pollution risk. This study aims to explore an environmental friendly process for dissociating toxic substances and recovering valuable components from SCC. Parameters of molten salt-assisted roasting and water leaching were optimized. A possible dissociation mechanism of toxic substances was proposed. Results showed that 99.12% of cyanide was decomposed and 96.63% of fluoride was leached under optimal conditions. The recovery route of fluoride was designed according to the solution equilibrium chemical calculation and the difference in solubility and particle size between the recovered products. Exhaust gas with a high concentration of CO and CO₂ was used for the carbonation of the leaching solution to recover cryolite. Effects of reaction conditions on precipitation mass and phase composition of recovered cryolite were investigated in detail. Characterization results indicated that the crystallinity and particle size of cryolite recovered under optimal conditions were extremely similar to those of commercial products. Finally, NaF and Na₂CO₃ were separated and recovered via evaporative crystallization combined with selective filtration. This proposed process with circular economy and green chemistry characteristics is expected to recover valuable components while minimizing environmental hazards of SCC.

Separating and recycling metal mixture of pyrolyzed waste printed circuit boards by a combined method

Waste printed circuit boards (WPCBs) contain a variety of valuable and hazardous materials. Recycling WPCBs is an important subject not only for environmental protection but also for sustainable development of resources. In this work, a new method combined low-temperature alkaline smelting with liquid-liquid phase separation is proposed to separate and recycle metal mixture in pyrolysis residue of WPCBs of mobile phones. During the low-temperature alkaline smelting process, amphoteric metals Al, Pb, Si, Sn, and Zn are firstly separated and recycled from the metal mixture with the separation rates of 99.5%, 81.6%, 97.8%, 88.4% and 95.7%, respectively. To separate the remaining metal mixture mainly containing elements Cu, Fe, Cr, Ni, Au and Ag, a liquid-liquid phase separation system is designed. As a result, the noble metals Au and Ag are concentrated in the copper-rich substance to form a high-value group, while the elements Ni and Cr distribute in the iron-rich substance. The iron-rich substance can be reused in the liquid-liquid phase separation process. In the super-gravity field, the recycling rates of the metals Au, Ag, Cr and Ni reach 98.1%, 99.8%, 95.6% and 75.4%, respectively. Furthermore, the iron-rich substance can be reused back to the liquid-liquid separation system. The copper-rich substance enriched by the noble metals can be efficiently recovered with low energy consumption and less pollution. This work provides an environmentally friendly and efficient route for separating and recycling the metal mixture in WPCBs.