Study on non-isothermal kinetics of the thermal desorption of mercury from spent mercuric chloride catalyst

Synergistic strengthening mechanisms of mechanical activation-microwave reduction for selective lithium extraction from spent lithium batteries

Carbothermal reduction of cathode materials is an effective method to selectively extract lithium carbonate, both mechanical activation and microwave heating can enhance thermal reduction of mixed electrode materials. However, the mechanism of enhanced lithium extraction has not been fully revealed. This study attempts to uncover the synergistic strengthening mechanisms of mechanical activation-microwave reduction from the aspects of material structure, dielectric properties, reduction kinetics and lithium recovery rate. Mechanical activation induces amorphization and structural defects. The enhanced dielectric properties of materials and the induced hotspots/arc plasmas are also responsible for the enhancement of the reduction reaction. The average dissociation activation energy in the activated sample is 18.0 kJ·mol^-1, which is 20.3 kJ·mol^-1 lower than that of unactivated sample. The model-free method reveals that the carbothermic reduction process can be divided into three stages: (I) initial stage (α < 0.4(0.6)): the activation energy gradually decreases with the formation of strong microwave acceptor-reduction products; (II) transitional stage (0.4(0.6) < α < 0.7): the increase in mass transfer resistance leads to gradual increase in activation energy. Mechanical activation shortens the transitional reaction stage; (III) later reaction stage (α > 0.7), the decrease in activation energy may be attributed to the enhanced microwave absorption and CO reduction. The model-fitting method reveals that after mechanical activation, the reaction kinetic changes from reaction-order model to Ginstling-Brounshtein diffusion model. The optimized lithium extraction process parameters were: activation 300 rpm for 1.5 h, reduction temperature 550 °C. The research results can provide theoretical support for the enhanced extraction of cathode materials.

Hg/Se/PbSO4 Recovery by Microwave-Intensified HgSe Pyrolysis from Toxic Acid Mud

The acid mud produced in the nonferrous smelting process is a hazardous waste, which mainly consists of elements Hg, Se, and Pb. Valuable metal (Hg/Se/Pb) can be recovered from acid mud by heat treatment. For safe disposal of the toxic acid mud, a new resource utilization technology by microwave roasting is proposed in this paper. The reaction mechanisms were revealed through thermodynamics and thermogravimetric analysis, which showed that the main reaction was the oxidative pyrolysis of HgSe in the process of roasting. Moreover, the mercury removal effects of acid mud by microwave heating and conventional heating were studied, the recovery rate of mercury by microwave heating for 30 min at 400 °C was 99.5%: far higher than that of conventional heating for 30 min at 500 °C (44.3%). This was due to the high dielectric constant of HgSe, as microwaves can preferentially heat HgSe and reduce the adsorption energy of HgSe on the surface of PbSO4 blocks, thus strengthening the pyrolysis process of HgSe and reducing energy consumption. The preferable prototyping technology for resource utilization of toxic acid mud should be microwave roasting. This study is of great significance for the realization of mercury pollution reduction and for green production of lead-zinc smelting.

A Novel Low-Temperature Fluorination Roasting Mechanism Investigation of Regenerated Spent Anode Graphite via TG-IR Analysis and Kinetic Modeling

Spent anode graphite, a hazardous solid waste discarded from the recovery of spent lithium-ion batteries (LIBs), had created social and environmental issues but has been scarcely investigated. Thus, a feasible, environmentally friendly, and economical process of low-temperature fluorination roasting and water leaching technology was proposed to regenerate spent graphite anodes. The results showed that the physical and chemical properties of regenerated graphite with a purity of 99.98% reached the graphite anode standard of LIBs and exhibited a stable specific capacity (340.9 mAh/g), capacity retention (68.92% after 470th cycles), and high initial Coulombic efficiency (92.13%), much better than that of waste carbon residue and similar to that of commercial graphite. Then the reaction mechanism and kinetic modeling of fluorination roasting of spent anode material was mainly explored by differential thermogravimetry and nonisothermal analysis methods. The results showed that the complexation and phase-transformation process of non-carbon valuable components in spent anode graphite occurred through three consecutive reactions in the 80-211 °C temperature intervals. The reaction mechanism of the whole process can be kinetically characterized by three successive reactions: third-order chemical reaction, Z-L-T eq, and second-order chemical reaction. Moreover, the thermodynamic functions of the fluorination roasting were calculated by the activated complex theory (transition state), which indicated the process was nonspontaneous. The mechanistic information was in good agreement with thermogravimetric-infrared spectroscopy (TG-IR), electron probe microanalysis, scanning electron microscopy, energy-dispersive spectrometry, and simulation experiments results.

Ultrasound and microwave-assisted recycling of spent mercuric chloride catalyst

It is urgent to develop a high-efficient process for recycling the spent mercuric chlorides catalyst (SMC) from vinyl chloride monomer (VCM) production with the implementation of the 'Minamata Convention on mercury'. A ultrasound and microwave-assisted technology were developed to treat SMC in this study. Firstly, organic carbon deposition was separated from SMC by pretreatment (ultrasonic-assisted ethanol extraction). The optimized extraction conditions were: ultrasonic time 2 h, ultrasonic power 700 W, extraction temperature 65°C, and liquid-solid ratio 7:1. Under these conditions, 90% of hazardous Cl-containing organics were separated from SMC. Then the pretreated SMC was treated by microwave heating for mercury removal. Residual mercury concentration of SMC decreased from original 1.33% to only 11.92 mg/kg at the preferred conditions of 500°C for 60 min and the treated SMC passed the Toxicity Characteristics Leaching Procedure (TCLP) test. Simultaneously, catalyst support activated carbon (AC) was regenerated with specific surface area increasing from original 263.85 to 627.5 m²/g. The organics from macropores and surface of AC was removed by pretreatment, intensifying the subsequent Hg removal and regeneration of AC as revealed by the comparative studies. Finally, SMC was subjected to water leaching for recovering metal values. 88.7% of Ba and 95.3% of Ce were leached with ultrasonic power 500 W and ultrasonic time 120 min. SMC was detoxified and valuable components Hg, AC, Ba, Ce were recovered by this new process, which may provide a new idea for industrial treatment of SMC.

Thermal behaviors, combustion mechanisms, evolved gasses, and ash analysis of spent potlining for a hazardous waste management

An unavoidable but reusable waste so as to enhance a more circular waste utilization has been spent potlining (SPL) generated by the aluminum industry. The combustion mechanisms, evolved gasses, and ash properties of SPL were characterized dynamically in response to the elevated temperature and heating rates. Differential scanning calorimetric (DSC) results indicated an exothermic reaction behavior probably able to meet the energy needs of various industrial applications. The reaction mechanisms for the SPL combustion were best described using the 1.5-, 3- and 2.5-order reaction models. Fluoride volatilization rate of the flue gas was estimated at 2.24%. The SPL combustion emitted CO₂, HNCO, NO, and NO₂ but SO_x. The joint optimization of remaining mass, derivative thermogravimetry, and derivative DSC was achieved with the optimal temperature and heating rate combination of 783.5 °C, and 5 °C/min, respectively. Interaction between temperature and heating rate exerted the strongest and weakest impact on DSC and remaining mass, respectively. The fluorine mainly as the formation of substantial NaF and CaF₂ in the residual ash. Besides, the composition and effect of environment of residual solid were evaluated. The ash slagging tendency and its mineral deposition mechanisms were elucidated in terms of turning SPL waste into a benign input to a circular waste utilization.

Mercury vapor adsorption and sustainable recovery using novel electrothermal swing system with gold-electrodeposited activated carbon fiber cloth

A novel electrothermal swing (ETS) system with gold-electrodeposited activated carbon fiber cloth (GE-ACFC) was developed to adsorb and sustainably recover low-concentration Hg⁰. GE-ACFC with an Au growth time of 1200 s displayed the largest Hg⁰ adsorption capacity and >90% removal efficiency. The Hg⁰ adsorption of GE-ACFC was dominated by physisorption via Au amalgamation. In contrast, Hg adsorption of untreated ACFC (RAW-ACFC) was mainly controlled by physisorption and chemisorption related to carbonyl groups. Nevertheless, both ACFCs could reach 100% ETS Hg⁰ regeneration. The Hg re-adsorption of GE-ACFC was stable, with efficiency >90% at different regeneration temperatures in three-cycle ETS experiments, but the Hg re-adsorption efficiencies of RAW-ACFC greatly decreased to only 60% after 250 ℃ regeneration, due to the formation of electrothermal hot spots in the ACFC. Because the thermal and electrical conductivity of GE-ACFC increased due to Au electrodeposition, the presence of electrothermal hot spots in GE-ACFC-1200s was minor. Simulation results showed that both pseudo-first-order and pseudo-second-order models fitted well to the desorption patterns of the GE-ACFC. Mass transfer model further suggested that intraparticle diffusion control was the rate-limiting step, with diffusion coefficients increased from the first to the third cycle for GE-ACFC.

Investigation on mercury removal and recovery based on enhanced adsorption by activated coke

This work is to systematically study the mercury-removal behavior of activated coke (AC), regeneration of spent AC by microwave treatment and subsequent recycling of Hg⁰. The powdery (AC) was obtained under coal-fired hot gas conditions in a drop-tube reactor. The adsorption mechanism and capacity of the AC for Hg⁰ removal in a H₂O + SO₂ + O₂ atmosphere were investigated. The regeneration of the AC by microwave heating and recovery of Hg⁰ were studied. The results showed that this AC preparation method can greatly simplify the process, and the AC's large surface area, developed pore structure, and abundant functional groups played a key role in the adsorption of Hg⁰. The adsorption mechanism and the optimum reaction conditions were determined, with a highest average Hg⁰-adsorption efficiency of 91% obtained at 70 °C in 3 h. Desorption of Hg⁰ was also studied, in which the alkaline-functional-group content and pore structure were enhanced, and S was detected by X-ray photoelectron spectroscopy in microwave-regenerated AC, which could improve the Hg⁰ removal efficiency increased to 96% after five adsorption/desorption cycles. The Hg⁰ could subsequently be recovered from the desorbed gas by condensation with an efficiency of 87.4% using ice-water.

Exploratory of immobilization remediation of hydroxyapatite (HAP) on lead-contaminated soils

This study was aimed to investigate the adsorption and fixation effects of hydroxyapatite (HAP) on lead-contaminated soil. According to the experimental results, the microstructure of hydroxyapatite was observed by a scanning electron microscope (SEM). Fourier transform infrared spectroscopy (FTIR) showed that OH^- and PO₄^3- were the main functional groups in HAP. Optimum adsorption conditions of Pb²⁺ were obtained: 0.2 g/L, adsorbent; initial solution pH of 5.5; and contact time of 120 min. The kinetic adsorption experiments were carried out with the initial lead solution concentrations of 50 mg/L, 150 mg/L, and 250 mg/L. The kinetics fitting was consistent with the pseudo-second-kinetic model, which indicated that the main process of HAP adsorption of Pb²⁺ was mainly controlled by surface reactions and chemical reactions. The adsorption isotherms had a satisfactory fit with the Langmuir model, which indicated that the adsorption of Pb²⁺ by HAP was a monolayer adsorption. According to the experimental results, ion exchange, phosphorus supply, precipitate, and complexation are the main immobilization mechanisms for soil remediation with HAP. In remediation of Pb²⁺-contaminated soil experiments, the adsorption rate of Pb²⁺ by HAP was significantly higher than that of non-HAP soil with increasing immobilization days. With the increasing addition of HAP, the weak acid soluble (WA), reducible (RED), oxidizable (OX), and water soluble (WS) are transformed into residue (RES). The application of HAP in contaminated soil effectively reduced the leachable and exchangeable Pb²⁺, indicating that HAP is a potential material for remediating environmental pollution with Pb²⁺.

Catalytic removal of mercury from waste carbonaceous catalyst by microwave heating

Waste carbonaceous catalyst (WCC) from vinyl chloride monomer (VCM) production is a potential environmental threat due to the mercury toxicity. Microwave heating (MWH) was used to decontaminate WCC. Treatment temperature had a stronger influence on mercury removal than that of treatment time while mercury removal was highly depended on treatment time at lower temperature. When WCC was treated at 350 °C for 60 min, 400 °C for 30 min and 450 °C or more for 10 min, leaching toxicity of mercury conformed to the US EPA standard. 99.98% of total mercury was removed and residual mercury concentration was only 4.5 mg kg^-1 when treated at 500 °C for 30 min. Soluble and exchangeable Hg and Hg combined with labile organics were more easily to be removed than that of Hg bound to crystalline Fe/Al oxides, Hg combined with non-labile organics and HgS. The removal limit for different mercury species may be achieved at 500 °C. Evaporation removal of mercury followed exponential decay model. Activation energy for mercury removal was reduced due to the catalytic effect of MWH. Removal mechanisms of mercury included thermal evaporation, breakdown of molecular bonds, selective stripping of carbonaceous impurities.