Dielectric properties and thermal behavior of electrolytic manganese anode mud in microwave field

Conventional and microwave-assisted acid pretreatment of tea waste powder: analysis of functional groups using FTIR

Tea waste powder (TWP) is one of the potential biomass waste to recover valuable chemicals and materials. The prime objective of this work is to investigate the role of acid pretreatment on TWP. Diluted acids (HCl, H₃PO₄, CH₃COOH, and H₂SO₄) were used to soak the TWP to understand the role of acids on bond cleavage and chemicals formation. One gram of TWP was soaked in 100 mL of diluted acids for 24 h. The soaked samples were further subjected to a hot air oven (temperature: 80 °C, duration: 6 h), orbital shaking (shaking speed: 80-100 rpm, duration: 6 h), and microwave irradiation (microwave power: 100 W, duration: 10 min) to understand the synergistic effects of acids and mode of exposure. The pretreated solid samples and liquid samples were analyzed using FTIR to understand the presence of functional groups. The mass loss of TWP after treatment significantly varied with the type of acid and exposure mode used. In the orbital shaker, the mass loss was varied in the following order: H₂SO₄ (36%) > CH₃COOH (32%) > H₃PO₄ (22%) > HCl (15%). In hot air oven, high mass loss was observed compared to orbital shaking [HCl (48%) > CH₃COOH (37%) > H₂SO₄ (35%) > H₃PO₄ (33%)]. The mass loss in microwave irradiation is lower (19 to 25%) with all acids compared to orbital shaking. In the solid samples, O-H stretching, C-H stretching, C=O stretching, C=C stretching, -C-O-, and -C-OH- functional groups were noticed. Similarly, C=O and C=C peaks and C-O and -C-OH peaks were noticed in liquid samples. Interestingly, microwave irradiation showed promising results in 10 min of pretreatment, whereas orbital shaking and hot air oven pretreatments require 6 h to achieve the same result.

Microwave-assisted recovery of lead from electrolytic manganese anode sludge using tartaric acid and NaOH

In this study, a new metallurgical system for treating electrolytic manganese anode sludge by microwave roasting and alkaline leaching system was developed, and the lead leaching behaviour was studied. The XRD results show that Pb in anode sludge is mainly in the form of PbSO₄ after microwave roasting at 850°C, as a result, the leaching rate of Pb is improved. The results show that the leaching rate of lead can reach 93.89% under the conditions of liquid-solid ratio of 7:1, leaching time of 30 min, leaching temperature of 40°C, and the concentration of sodium hydroxide of 8%. The addition of tartaric acid can further improve the lead leaching rate, FT-IR analysis showed that the coordination form of lead and tartaric acid. Lead and tartaric acid ions (L) form three coordination compounds, PbL, Pb₂L₂ and Pb₂L₃, which can only exist in alkaline solution.

An atmospheric microwave plasma-based distributed system for medical waste treatment

Inadequate handling of infectious medical waste may promote the spread of the virus through secondary transmission during the transfer process. Microwave plasma, an ease-of-use, device-compact, and pollution-free technology, enables the on-site disposal of medical waste, thereby preventing secondary transmission. We developed atmospheric-pressure air-based microwave plasma torches with lengths exceeding 30 cm to rapidly treat various medical wastes in situ with nonhazardous exhaust gas. The gas compositions and temperatures throughout the medical waste treatment process were monitored by gas analyzers and thermocouples in real time. The main organic elements in medical waste and their residues were analyzed by an organic elemental analyzer. The results showed that (i) the weight reduction ratio of medical waste achieved a maximum value of 94%; (ii) a water-waste ratio of 30% was beneficial for enhancing the microwave plasma treatment effect for medical wastes; and (iii) substantial treatment effectiveness was achievable under a high feeding temperature (≥ 600 °C) and a high gas flow rate (≥ 40 L/min). Based on these results, we built a miniaturized and distributed pilot prototype for microwave plasma torch-based on-site medical waste treatment. This innovation could fill the gap in the field of small-scale medical waste treatment facilities and alleviate the existing issue of handling medical waste on-site.

Microwave plasma torches for solid waste treatment and vitrification

Conventional thermal plasma technology used in the treatment of solid waste has a high demand for power and a high rate of heat loss during solid waste treatment. We developed a novel approach for treating and vitrifying solid waste with a low-power microwave plasma torch (MPT). Based on theoretical thermodynamic equilibrium calculations, we studied the melting temperature of the residual ash and achieved vitrification of the residual ash with an MPT by adding specified ratios of discarded glass scraps. Thermocouples and a gas analyzer were used to characterize the temperature variations and gas concentrations in the plasma treatment chambers, respectively. An organic elemental analyzer and X-ray fluorescence (XRF) analyses were used to determine the chemical proportions of the solid waste residues. The morphologies of the residues and vitreous material were analyzed using scanning electron microscopy (SEM). The results showed that the microwave plasma treatment process converted 96 wt.% of the solid wastes into nonpolluting gases, leaving a residue of pure carbon and inorganic powder. Through theoretical calculations and experiments, atmospheric MPTs with power levels less than 10 kW were identified as realistic means for treating and vitrifying solid wastes.

The preparation and properties of iron-walnut shell powder microelectrolytic spherical fillers

In order to solve the issues of caking, loss, and effluent color reversion in the application of traditional microelectrolysis, the iron-walnut shell powder microelectrolytic spherical filler was developed in this paper. The filler was prepared by walnut shell powder, iron powder, sodium silicate, and sodium humate activated by ZnCl₂ as raw materials and calcined at high temperature. The effects of the mass ratios of Fe to walnut shell powder, sodium silicate content, sodium humate content, calcination temperature, and time on the removal rate of methylene blue by the spherical fillers were investigated, so as to determine the optimal preparation conditions of the spherical fillers. The pore-forming structure and the composition of the spherical fillers were also analyzed by an X-ray diffractometer (XRD), a scanning electron microscope (SEM), and an energy spectrometer (EDS). The results show that the optimal preparation conditions for the spherical fillers of 5 mm are as follows: the mass ratio of iron powder to walnut shell powder treated by 15% ZnCl₂ is 1:1, sodium silicate is 15%, sodium humate is 20%, the calcination temperature is 800 °C, and the calcination time is 3 h. Compared with the conventional microelectrolysis, the removal rate of methylene blue by the spherical fillers can finally reach the same level as it did, and the phenomena of the filler hardening and clogging can be avoided.

Investigations on the Thermodynamics Characteristics, Thermal and Dielectric Properties of Calcium-Activated Zinc-Containing Metallurgical Residues

An activate pretreatment of zinc-containing metallurgical residues were proposed by adding CaO and introducing microwave heating approach into the CaO activation pretreatment process to realize the conversion of refractory ore phases into pre-treated ore phase. Thermodynamic characteristics analysis indicated that adding CaO can realize the conversion of refractory ore phases, with the same effect as the carbon additives. Thermal conductivity properties analysis denoted that the thermal conductivity properties of ZnS and ZnFe2O4 were relatively poor. Meanwhile, the thermal conductivity properties of the residues sample added with 25% CaO were significantly superior to the residues added with other CaO contents, with the maximum specific heat value of 1.348 J/g·K at 350 °C. Dielectric properties analysis highlighted that adding CaO with the dielectric constant properties significantly higher than that of other substances can enhance the microwave absorption capacity of zinc-containing residues. The decrease in dielectric loss and loss tangent value with the increase of temperature and the residues having large microwave penetration depth guaranteed to obtain better uniformity of microwave heating. Furthermore, adding 25% CaO promoted the microwave penetration depth of the residues sample increased in the range of 300-500 °C. This work can lay a theoretical research foundation for solving the key difficulty for efficient Zn recovery from complex zinc-containing metallurgical residues.

Study on the mechanism and kinetics of manganese release from waste manganese ore waste rock under rainfall leaching

Manganese released from the piled manganese ore wastes is a great threat to the local ecosystem and human health. The mechanism and dynamic characteristics of manganese release from the manganese ore wastes were studied based on the static and dynamic experiments. The concentration of manganese in the leaching solution under the intensive state is twice that resulted from the static state; the manganese release from the waste rock increased with the increase of the solid-liquid ratio and reached 922.3 mg/L when the solid-liquid ratio was 1:5. When the particle size of waste rock was less than 180 μm, the release amount of manganese was the largest and reached 491.3 mg/L. When the pH was 7 and the rainfall intensity was 80 mL/h, the increase of leaching time contributed to the rapidly decreased amount of manganese released, and the leaching process reached equilibrium gradually. The cumulative release of manganese increased with the increase of rainfall duration. In the dynamic leaching process, the change of pH and EC of the leachate had nothing to do with the initial pH of leaching agent but has a close relationship with the hydrolysis of minerals in waste. According to the experimental results, it was found that the double constant equation model fitted the kinetic process of release process better. The purpose of this study was to provide a scientific basis for the assessment and control of manganese pollution in soil and groundwater in manganese mining area.

Ultrasonication-Assisted Preparation of a Mn-Based Blast Furnace Slag Catalyst: Effects on the Low-Temperature Selective Catalytic Reduction Denitration Process

Reducing costs and improving performance have always been hotspots in the field of catalyst research. In order to control the NO x in the low-temperature flue gas of nonpower industries, this paper studies the denitration performance of the ultrasonication-assisted preparation of Mn-based blast furnace slag selective catalytic reduction (SCR) low-temperature denitration catalysts. The catalyst was characterized by FT-IR, XRD, and SEM. The study found that ultrasound assistance can make the active components on the catalyst surface more uniformly dispersed and improve the catalytic activity of the catalyst. Under conditions of 80 W ultrasonic power and 20 min ultrasonic time, the denitration performance of the Mn-based blast furnace slag catalyst is optimal, and the NO removal rate is 2.5 times that of the unsonicated catalyst. This work clarified the mechanism of the effect of ultrasonic assistance on the Mn-based blast furnace slag catalyst and at the same time realized the utilization of solid waste resources and air pollution control.

Bioleaching of manganese from a low-grade pyrolusite ore using Aspergillus niger: Process optimization and kinetic studies

Low-grade metal resources generated during different mineral processing activities are increasing while there are not many economic and environmentally friendly techniques to manage them. There is no viable technique for the manganese extraction from low-grade ores as the conventional procedures are costly and environmentally unfriendly. In this research, the D-optimal response surface methodology has been used to optimize the bioleaching parameters. Varied contact methods (one-step, two-step, and spent medium), nutrition sources (sucrose and glucose), and pulp densities (1 g.L^-1 to 10 g.L^-1) were used in different experiments having been done in 30 days using Aspergillus niger. A maximum recovery of over 80% of Mn was achieved based on the acidolysis, complexolysis, and redoxolysis leaching of the organic acids produced by the fungi under the optimum condition; a two-step approach, in a glucose medium, and with a pulp density of 1 g.L^-1. A kinetic study was also performed and revealed that the leaching mechanism was a mixed one which consisted of two stages (diffusion through the liquid film and a chemical reaction) for the first 12 day period, and a mechanism of diffusion through the product layer for the rest of the experiment.

Lead release kinetics and film transformation of Pb-MnO2 pre-coated anode in long-term zinc electrowinning

Lead pollution precaution caused by lead-based anode corrosion is a hot and challenging issue for zinc electrowinning. A novel functional lead-based anode (MnO₂ pre-coated anode-MPA) was precisely fabricated and its long-term performances were studied compared with typical Pb-1%Ag anode (TPA). Results indicated that MPA posed excellent effects on synergistic inhibiting lead dissolution and reducing hazardous pollutants generation, and decreasing the lead content of zinc products by 81%. Further, the underlying mechanism of film growth and transformation in structure, composition and crystal phase, the migration and distribution of lead and anode slime during electrolytic, were clarified in-depth. Dynamic material flow analysis confirmed that MPA reduced the entire lead migration amount by over 92% compared with TPA. The compact multilayer structure of the MPA film and self-reparation effects of local structure provided better and persistent protection for the lead matrix, which greatly retarded the high-speed corrosion of lead anode. Compared with α-MnO₂ in TPA, the formation and maintenance of γ-MnO₂ in MPA accelerated the oxygen evolution reaction and inhibited the anode slime generation. This finding provides new insights in pollution precaution and control by designing and tuning new functional anode in hydrometallurgy process.

Kinetics characteristics and microwave reduction behavior of walnut shell-pyrolusite blends

Combining biomass pyrolysis with microwave heating technologies provides a novel and efficient approach for low-grade pyrolusite reduction. The microwave reduction behavior and pyrolysis kinetic characteristics of walnut shell-pyrolusite blends were explored. Results indicated the optimal reduction parameters were: reduction temperature of 650 °C, holding time of 30 min, M_bio/M_ore of 1.8:10, and microwave power of 1200 W. The co-pyrolysis characteristics of the blends included four stages: dehydration, pre-pyrolysis, intense pyrolysis and reduction, and slow pyrolysis and reduction. Fitting analysis based on Coats-Redfern method revealed that chemical reaction was the control step of the process of reducing pyrolusite by biomass, which the finding matched to the isothermal kinetic analysis results determined through unreacted shrinking nuclear model. The activation energies and pre-exponential factors were determined at 5.62 kJ·mol^-1-16.69 kJ·mol^-1 and 0.0426 min^-1-0.515 min^-1. The work provides sound references for promoting the industrial application of the combined method on minerals reduction.

C,N-codoped MoSi2 ceramic with excellent heat resistance for microwaves absorption application

Microwave absorption (MA) materials with high heat resistance have a wide range of applications in many fields. In this work, a C,N-codoped MoSi2 ceramic was prepared via a facile solid-phase reaction method and its MA properties was investigated. On the one hand, the results indicate that this ceramic possesses excellent heat resistance and the weight of the MoSi2 is almost constant when the temperature is lower than 800°C. On the other hand, this ceramic shows good MA performance when the filler loading ratio increases to 30 vol%, the value of reflection loss (RL) could reach to -17.70 dB at 7.44 GHz with the thickness of 2.0 mm and the effective electromagnetic absorption bandwidth (RL below -10 dB) could reach to 1.88 GHz (9.28-11.16 GHz) with the thickness of 1.5 mm. Multi-polarization resonance loss is considered as the predominant attention mechanism on the MA performance of this MoSi2 ceramic. This research provides a new idea for understanding resonance mechanism and greatly expands the application scope of MoSi2 ceramic in MA area.

Microwave catalyzed carbothermic reduction of zinc oxide and zinc ferrite: effect of microwave energy on the reaction activation energy

Recently, more attention has been paid to the use of microwave (MW) energy in accelerating chemical reactions. The effect of microwave energy on the reduction of zinc oxide and zinc ferrite was investigated. The results indicated that the temperatures required to initiate zinc oxide and zinc ferrite reduction under MW heating were 550 and 450 °C, respectively, while under conventional thermal (CT) heating, were 950 and 850 °C, respectively. Apparently, the MW reaction had a negative standard Gibbs free energy (ΔG) at a lower temperature (∼400 °C) when compared to the CT reaction. Additionally, the activation energy (E _a) substantially decreased from 223.7 and 221.1 kJ mol^-1 under CT heating to 64.8 and 32.9 kJ mol^-1 under MW heating for Zn oxide and zinc ferrite, respectively. The enhancement in zinc reduction under MW energy was due to the rapid and bulk heating phenomena of MWs as well as the interactions occurring between the electromagnetic MW pattern and the molecules of heated materials.