H2SO4 Concentration Research Articles

Continuous multiphase Bunsen reactor of iodine-sulfur thermochemical water splitting cycles for hydrogen production: Experimental, Modelling and Design Insights

Thermochemical water splitting cycles (TWSCs) hold promise for sustainable H2 production in future energy systems, with Iodine-Sulfur (IS) and Nickel-Iodine-Sulfur (NIS) processes standing out as efficient options. The core of both these processes lies in the Bunsen reaction, underscoring the need to identify optimal operational conditions and plant solutions for this crucial reaction section. Thus, in this study, a continuous counter-current flow Bunsen reactor fed with 5 NL h−1 of SO2 was constructed and tested. An inventive aspect was the introduction of solid I2 pellets from the top to enhance saturation at the liquid-liquid interface between sulfuric and hydriodic product phases, promoting segregation. This ingenious design achieved nearly complete SO2 conversion and optimal H2SO4 and HI concentrations in the respective product phases, while avoiding undesirable byproduct formation. The study further advanced the modelling of the experimental reactor by employing innovative kinetic and liquid-liquid equilibrium description approaches, and demonstrating exceptional validation accuracy of R2 = 0.9988. Additionally, with the aim of obtaining design charts as a potential tool for Bunsen reactor design, a microscopic model describing the gas phase trend along bubble or packed Bunsen-type reactors was developed. The model’s dimensionless analysis revealed a characteristic times ratio (Bu) comprehensively describing all the phenomena occurring to the gas phase within the Bunsen reaction section, and identified an optimal Bu value of 0.4 for pure SO2 gas inlet. Overall, the study's innovative strategies and models contribute significantly to the advancement of Bunsen reactor engineering.

Recovery of Valuable Metals from Polymetallic Refractory Concentrate by a Sulfuric Acid Curing and Leaching Method

Sulfuric acid curing and leaching is a promising technology for treating refractory ores. In this work, a refractory concentrate containing 3191 ppm uranium (U), 2135 ppm niobium (Nb), and 0.7% rare earth minerals (REMs) went through two stages: curing by high-concentration H2SO4 and leaching by low-concentration H2SO4. We investigated the behavior of those valuable metals during the two stages. For both curing and leaching, the operating parameters include the acid-to-solid ratio, time, temperature, and H2SO4 concentration. The recovery for U, Nb, and REMs was as high as 95%, 86%, and 73.5% using a curing acid-to-solid ratio of 1:1, curing temperature of 200 °C, curing time of 1 h, H2SO4 concentration of 98%, leaching liquid-to-solid ratio of 4:1, leaching time of 2 h, leaching temperature of 60 °C, and leaching H2SO4 concentration of 5 g/L. A “sulfuric acid curing–leaching-U extraction by N235–Nb recovery by resin adsorption–REMs’ recovery by resin adsorption” method was implemented, where the overall U, Nb, and REMs’ recovery reached 93.1%, 84.5%, and 69.6%, respectively.

Electrodialysis of Lithium Sulfate Solutions from Hydrometallurgical Recycling of Spent Lithium-Ion Batteries

Recycling lithium-ion battery materials is a crucial step to achieving a circular economy. A hydrometallurgical recycling process of spent lithium-ion battery materials involves leaching by acids and precipitation by bases. A large amount of Li2SO4 leachate solution is generated at the end of the recycling process. Such Li2SO4 solution can be separated by electrodialysis (ED) to form H2SO4 and LiOH solutions, which are reused for leaching and precipitation, respectively, making the recycling a closed-loop process. In this study, an ‘electrodialysis stack constructed with bipolar membranes’ (EDBM) is built with repetitive unit cells consisting of a bipolar membrane, anion exchange membrane (AEM), and cation exchange membrane (CEM), see Fig. 1(a). The effective area for each membrane is 121.8 cm2 (70mm×174mm). The gap between these membranes is 0.8mm, with a mesh inserted to allow flow mixing. All experiments were operated at constant current mode. The feed, acid, and base solutions were recirculated through their tanks at initial concentrations of 1.1, 0.1, and 0.1 mol/L. The ion concentrations of the solutions were measured with Inductively coupled plasma mass spectrometry periodically. The weight and the pH value of the solution tanks were also recorded.The trend of concentration variations of the present stack with up to 6 cell pairs is found to be consistent with that observed with a three-compartment configuration in our previous study [1]. The concentration of H2SO4 increases with time linearly, whereas that of LiOH levels off due to a significant electro-osmosis that drags water through the CEM into the concentrate channel. The present setup differs from the three-compartment design in the bipolar membrane, which splits water to produce H+ and OH- at the internal interface, eliminating the formation of bubbly flows in the unit cell. It is found that the voltage loss across the bipolar membrane ca. 1V accounts for a major part of the unit cell of about 1.27V.The effects of current density, number of cell pairs, and flow rates on the overall stack performance are investigated. Increasing the stack current density accelerates the separation process at the cost of higher voltage losses. It is found that increasing the number of cell pairs can improve the overall production efficiency and reduce energy consumption per cell pair. For instance, at 800 A/m2, the specific energy consumption for LiOH production is reduced to 3.40 kWh/kg with five cell pairs versus 5.83 kWh/kg with two cell pairs. It is also found that increasing the solution flow rate can result into the increase of LiOH solution concentration, ion recovery rate, and current efficiency (Fig. 1b), although the impact of flow rate is low.A small EDBM stack is constructed and tested for Li2SO4 separation in the present study. This work demonstrates that EDBM can be a simple and energy-saving alternative to the current chemical precipitation method to produce LiOH from Li2SO4. The present study provides new insights into optimal operation and design for Li2SO4 EDBM. The findings are helpful in determining how the stack can be scaled up for practical application. Furthermore, a 2D multiphysics model is currently under development to elucidate the coupled transport processes within the unit cell. The model will be validated with experimental data of the present setup. Figure 1

Co-Removal of Fe/V Impurity in H2TiO3 Synthesized from Ti-Bearing Blast Furnace Slag.

Ti-bearing blast furnace slag (TBFS) can be converted to impurity bearing TiOSO4 solution for TiO2 pigment production. However, the H2TiO3 (MTA) hydrolyzed from the solution has too high Fe/V impurity to meet the standard for TiO2 pigment. In this study, we found that Fe3+ and V3+ were easily hydrolyzed and entered the MTA lattice, and hence could not be removed by washing. Furthermore, Fe/V was hard to co-remove by the traditional reduction method. Therefore, the Fe/V non-hydrolysis condition (Ti3+ = 0.01 M, F = 3.0, T = 130 °C; Ti3+ = 0.01 M, F = 3.5, T = 150 °C) was determined by thermodynamic calculations. However, at these conditions, the Ti hydrolysis ratio was low or the reaction time was long. Therefore, a new two-step hydrothermal hydrolysis process was proposed. Step 1 (130 °C, 2 h) ensured the non-hydrolysis of V3+, and Ti was partially hydrolyzed to increase the H2SO4 concentration. Step 2 (150 °C, 2 h) ensured a high Ti hydrolysis ratio (>0.95) and short total reaction time (4-6 h). Finally, a high-purity MTA was obtained (Fe = 21 ppm, V = 145 ppm). These results provide new insights into the control of the hydrolysis of impurity ions in solutions and help to optimize the process of TiO2 pigment preparation from TBFS.

Solvent extraction of scandium from leaching solution of red mud roasted with ammonium sulfate using D2EHPA/TBP

Red mud is an important secondary resource for scandium production. The red mud leaching solution containing scandium in this study was derived from environmentally friendly ammonium sulfate roasting and water leaching process. A synergistic extraction with a mixture of di(2-ethylhexyl) phosphate acid (D2EHPA) and tributyl phosphate (TBP) for recovery of scandium from red mud leaching solution is proposed. The effects of D2EHPA concentration, H2SO4 concentration, rare earth elements, dosage of TBP, phase ratio (A/O), contact time and H2O2 concentration on scandium extraction were investigated. The results show that more than 99% scandium is extracted under the optimal conditions while Fe, Al, Ti, Ca and rare earth elements (Ce, Y, La, Nd, Er, etc.) are hardly extracted. The stripping efficiency of Sc reaches above 92.37% under the optimal stripping conditions of 5 mol/L NaOH with an A/O of 1 at 90 °C for 30 min. The proposed technology could provide an effective method for extraction of scandium from red mud leaching solution.

Hydrotreatment of Eucalyptus sawdust: The influence of process temperature and H2SO4 catalyst on hydrochar quality, combustion behavior and related emissions

Hydrothermal carbonization (HTC) presents a promising avenue for waste-to-energy conversion, owing to its cost-effectiveness, low emissions, and adaptability. This study employed HTC to convert Eucalyptus grandis sawdust residues into hydrochars (HC). HTC was conducted for 60 min in a high-pressure steel reactor within glycerin thermal bath equipment under different temperatures (150, 170, 190, and 220 °C) and reaction mediums (water and H2SO4 concentrations of 0.1 and 0.2 mol L–1). The resulting HC underwent thorough analysis, encompassing proximate, ultimate, and morphological assessments and investigations into combustion behavior and potential emissions. Moreover, a new indicator, the hydrotreatment severity index (HSI), is proposed to provide a standardized measure of hydrotreatment severity and property predictions. Higher temperatures and increased H2SO4 concentrations were found to reduce solid yield (ranging from 43.21 % to 90.72 %) and induce morphological changes, including an abundance of carbon microspheres, particularly at higher acid concentrations. These modifications substantially improved higher heating values (HHV), up to 34 % (HHV of 25.72 MJ kg−1 achieved at 220 °C with 0.2 mol L–1 H2SO4) compared to raw feedstock. Interestingly, the intermediate temperature of 190 °C contributed significantly, enhancing HHV by 33 %. All HTC processes greatly enhanced the combustion performance and ignitability of hydrochars. The most favorable reactivity (S = 7.61E7 %2 min−2 °C−3) was observed at 220 °C with 0.2 mol L–1 H2SO4, along with an estimated CO2 emissions of 2,058.46 kg per ton of hydrochar. These results underscore the efficacy of the simplified methodology and reactor system, emphasizing the critical roles of H2SO4 as catalyst and temperature in shaping the characteristics and performance of hydrochars from a representative Brazilian residue, Eucalyptus grandis.

Upgrading Pyrolytic Residue from End-of-Life Tires to Efficient Heterogeneous Catalysts for the Conversion of Glycerol to Acetins.

Recovered carbon blacks (rCBs) produced from end-of-life tires using pyrolysis were transformed into solid acid catalysts for the synthesis of acetins, i.e., products with a wide spectrum of practical applications. Tuning the chemical properties of the surface of samples and introducing specific functional groups on the rCBs were achieved through carbon functionalization with concentrated H2SO4. The initial and modified rCBs were thoroughly characterized using techniques such as elemental analysis, potentiometric back titration, thermogravimetric technique, scanning and transmission microscopy, X-ray photoelectron spectroscopy, etc. The catalytic activities of the samples were measured via batch mode glycerol acetylation performed at 110 °C and compared to the catalytic performance of the functionalized commercial carbon black. The modified rCBs were found to show a significant catalytic effect in the tested reaction, giving high glycerol conversions (above 95%) and satisfactory combined yields of diacetins and triacetin (~72%) within 4 h; this behavior was attributed to the presence of -SO3H moieties on the surface of functionalized rCBs. The reusability tests indicated that the modified samples were catalytically stable in subsequent acetylation runs. The obtained results evidenced the feasibility of using end-of-life tires for the production of effective acid catalysts for glycerol valorization processes.

Exploring the Conductivity, Solubility, and thermodynamic properties of sulfonated waste styrofoam through experimental and theoretical studies

In this work, sulfonated WPS (SWPS) was synthesized using concentrated sulfuric acid (Con. H2SO4) at various concentrations (0.01–0.05 mol.dm−3) and temperatures (313 K, 333 K, and 353 K) and the structure was confirmed by various analytical techniques. The degree of sulfonation (DS) values was determined using cation exchange capacity (CEC), 1H NMR, and UV–visible spectroscopy, and to investigate the influence of temperature and various concentration of H2SO4 during the sulfonation reaction. The conductivity of the SWPS was calculated and found to range from 0.23 × 10−6 S.cm−1 to 40.2 × 10−6 S.cm−1. The theoretical solubility parameters of SWPS were determined and compared with experimental values. Using Frenkel-Erying equation, Thermodynamic parameters of SWPS was calculated and also evaluated the correlation between the intrinsic viscosity (η) and viscous flow activation free energy (ΔGv). Our findings contribute to a better understanding of the sulfonation reaction of WPS and the potential applications of SWPS as a functional material.

SURFACE QUALITY AND CORROSION RESISTANCE OF 316L STAINLESS STEEL ELECTROPOLISHED USING PHOSPHORIC – SULFURIC ACIDS

Electropolishing is an electrochemical surface finishing technique. It is commonly applied to equipment that requires a gleaming finish. This surface property is frequently required in 316L stainless steel (SS) medical implants. Electropolishing removes a thin layer from the metal's surface through electrochemical processes. This results in a very clean, smooth, and bright metal surface. The process parameters, such as electrolyte solution, electrical current, and electropolishing time, influence surface roughness and glossiness. The dissolution of metallic ions during the process may also affect the corrosion resistance of the treated material in addition to producing a shiny surface. This study investigated the surface glossiness, surface roughness, and corrosion of electropolished 316L SS. Electropolishing experiments on 316 SS were carried out using various H3PO4 (50%) and H2SO4 (32%) electrolyte solution compositions. The influences of electrolyte solution composition, electric current, and electropolishing time were studied. The results showed that increasing the H2SO4 content of the mixture and electropolishing the 316L SS for a longer period of time improved the surface roughness and glossiness. Under 10 Amp electric currents, the best surface glossiness was discovered. A corrosion test revealed that the electropolishing produced a Cr and Ni-rich layer that improved the corrosion resistance of the samples.

Efficient recovery and regeneration of FePO4 from lithium extraction slag: Towards sustainable LiFePO4 battery recycling

The recovery and regeneration of FePO4 from lithium extraction slag (LES) are crucial steps in the closed-loop recycling of waste LiFePO4 batteries. This necessity arises despite the low commercial value of LES, as conventional recovery methods are cost-prohibitive, leading to insufficient attention to LES recovery. The recent surge in retired batteries has resulted in a significant accumulation of LES, posing serious environmental concerns, however, the high content of Fe and P resources in LES presents a potential value. The study begins with a theoretical analysis, concluding that the reduction leaching of LES is a feasible approach. By integrating this with the Fe powder synthesis route for FePO4, it is found that efficient leaching of LES can be achieved using a low concentration of H2SO4, thereby significantly reducing the recovery cost. This method not only efficiently utilize LES but also alleviates environmental pressure. The thermodynamic analysis sheds light on the mechanism of LES leaching and FePO4 synthesis, confirming the feasibility of each reaction. Optimal leaching conditions (1.4 times the stoichiometric amount of H2SO4, 1.5 times the amount of Fe powder, an L/S ratio of 5 mL/g, and a reaction duration of 150 min at 60 °C), under which the leaching rates of Fe and P reached 94.66% and 98.23%, respectively. Additionally, the study introduces a method to remove the main impurity Al in LES, achieving a removal rate of over 99%, reducing its concentration to below 100 ppm. Other impurities, such as Ti, Mn, and Zn, also meet the Chinese Chemical Industry Standard (HG/T 4701-2021). Comparative analyses were conducted between regenerated R-FePO4, commercial C–FePO4, and untreated LES (r-FePO4). LiFePO4 materials derived from these sources were synthesized, followed by an evaluation of their electrochemical performance. The regenerated R-FePO4 matched the quality of commercial FePO4 and significantly outperformed r-FePO4. Additionally, the process proposed in this study yields significant economic benefits, estimating a profit of 719.6 $/t of recycled LES. This research proposes a cost-effective and efficient method for regenerating high-quality battery-grade FePO4 from LES, offering a significant contribution to the closed-loop recycling of waste LiFePO4 batteries.

Hybrid membrane distillation and ion exchange process for resources recovery from mining wastewater

This work aims to achieve a process intensification through the hybridization of direct contact membrane distillation (DCMD) and ion exchange (IX) to increase resource recovery (water, H2SO4, Copper (Cu), Nickel (Ni), and Cobalt (Co)) from a gold mining wastewater. An increase in maximum recovery rate was achieved by the DCMD in the hybrid configurations – from 34 to 52 %. Recovery of Cu, Ni and Co is best achieved by NH4OH (4 mol/L), however with considerable carryover. Conversely, selective recovery can be achieved by different eluting agents H2SO4 0.1 mol/L to recover Co (~80 %), H2SO4 1 mol/L to recover Ni (~47 %), and NH4OH 4 mol/L to recover Cu (~26 %), with a lower carryover of other ions (<14 %). Scale-up calculations showed that the Amberlyst A26 mass required for the hybrid IX (batch) is 2.4× lower than the Dowex M4195, which resulted from the exchange capacity and selectivity. Conversely, the estimated Amberlyst A26 mass required for the hybrid IX (fixed-bed) was higher due to the higher initial concentration of H₂SO₄. Lastly, CapEX ranged from 0.2 to 5.6 M US$, and OpEX from 2.61 to 11.42 US$/m3. Considering earnings with the recovered resources the hybrid fixed-bed configurations are a clear choice due to the lowest payback times (<4 years).

Optimized yield of fermentable sugar from chemical hydrolysis of rice straw for application in ethanol fermentation

Lignocellulosic biomass is a rich source of carbohydrate polymers with cellulose and hemicellulose being the two primary carbohydrates made up of glucose and xylose monomers. These monomeric sugar molecules act as precursor molecules for ethanol production by the microbial fermentation process. Rice straw is a potent lignocellulosic feedstock for ethanol production, but its utilization on an industrial scale still faces significant challenges. The main obstacle lies in the chemical pretreatment process which needs to be designed optimally to enable a smooth supply of biomass-based fermentable sugar for ethanol businesses in a sustainable and cost-effective manner. The application of response surface curve analysis was made utilizing the Minitab software-based design of experiments which have demonstrated promising results in obtaining an optimized yield of fermentable sugar from the chemical hydrolysis of rice straw. The present study aimed to increase the fermentable sugar yield from chemical pretreatment of rice straw using Minitab computer software-based design of experiments. The optimal level of pretreatment variables was determined using Minitab 17 software-based analysis of the response surface curve to achieve a maximized release of fermentable sugar at 348.20 milligrams/gram of solid pretreated biomass. This study identified the corresponding optimum operating levels for each variable as (a) biomass solid loading rate (15% w/v), (b) H2SO4 concentration (12% v/v), (c) pretreatment reaction time (30 minutes) and (d) temperature (100°C).

Rare earth dissolution from polishing powder waste in H2O2-H2SO4 system: Condition optimization and leaching mechanism

Recycling high-grade rare earth polishing powder waste (REPPW) is crucial for protecting the environment and sustainable utilization of rare earth resources. Leaching REPPW in the eco-friendly H2O2-H2SO4 suffers from the lack of understanding of the leaching mechanism. Here, the leaching mechanism was investigated under the optimal leaching conditions, i.e., T = 353 K, L/S ratio = 40 mL/g, H2SO4 concentration = 4 mol/L and initial H2O2 concentration = 0.8 mol/L, by which leaching efficiency of Ce and La reached 99.3% and 99.1%, respectively. Furthermore, pseudo steady state kinetic analysis suggested a five-step leaching mechanism involving CeO2 hydroxylation, H2O2-CeO2 coordination, ≡CeIII−H2O2 deprotonation, ≡CeIV−HO2− reduction and ≡CeIII−HO2∗ protonation, which was verified by the agreement between experimental data and the proposed reaction model. This work proposes a leaching mechanism of REPPW and would help develop an effective and sustainable recycling process for rare earth secondary resources.

The Optimization of Initial Treatment of Seaweed Ulva reticulata Using CEM Synthesizer Method for Bioethanol Production

Research has been carried out on the optimization of initial treatment and hydrolysis using CEM microwave synthesizer and the production of bioethanol from Ulva reticulata seaweed. Optimization in the initial treatment was carried out by varying the concentration of HCl and H2SO4 (each in 1; 3; 5; and 7%), variations in time (30; 40; 50; and 60 minutes), temperature (100; 150; 200 and 250 °C), and electrical power (100; 150; 200; and 250 W). Fermentation was carried out anaerobically at 10% inoculum concentration and a production time of 6 days. Characterization of reducing sugar using DNS method and characterization of ethanol using GC-FID and HPLC. The results of the initial lignocellulosic analysis obtained the lignin content of 10.03%, cellulose 14.38% and hemicellulose 22.29%. After the initial treatment, the lignin content decreased to 3.86%, while cellulose increased to 24.50% and hemicellulose to 41.57%. The reducing sugar content produced using HCl is 97.10 g/L at optimum temperature 200 °C, for 60 minutes, using 7% concentration of HCl and 200 W of power, while the optimum reducing sugar content using H2SO4 is 76.40 g/L at optimum temperature 200 °C, time for 50 minutes, using 3% concentration of H2SO4 and 200 W of power. Production of bioethanol through fermentation and distillation processes obtained a bioethanol level of 43.89% (GC) or 18.89% (HPLC) for optimum conditions using H2SO4, whereas for optimum conditions using HCl, the bioethanol level is 44.29% (GC) or 18.09% (HPLC).

A highly efficient, rapid, room temperature synthesis method for coal-based water-soluble fluorescent carbon dots and its use in Fe3+ ion detection

We report a method for the of coal-based fluorescent carbon dots (CDs) at room temperature using a mixture of hydrogen peroxide (H2O2) and formic acid (HCOOH) as the oxidant instead of concentrated HNO3 or H2SO4. The CDs have an excitation dependent behavior with a high quantum yield (QY) of approximately 7.2%. The CDs are water soluble and have excellent photo-stability, good resistance to salt solutions, and are insensitive to pH in a range of 2.0-12.0. The CDs were used as a very sensitive probe for the turn-off sensing of Fe3+ ion with a detection limit as low as 600 nmol/L and a detection range from 2 to 100 μmol/L. This work provides a way for the high value-added utilization of coal.

A Study of the Corrosion Behavior of Engineering Mild Steel in Acidified and Hydrolyzed Extracts of Zingiber Officinale (Ginger)

We report the use of weight loss technique in the study of the corrosion behaviour of engineering mild steel in acidified and hydrolysed corrosion media using Zingiber officinale (Ginger) extracts. Cylindrical steel rods were cut into corrosion coupons of predetermined dimensions of 15 mm by 10mm diameter. The coupons were then weighed before being suspended in situ in fours into beakers containing 10 mL, 15 mL and 20 mL of the extracts in 0.5 M and 1.0 M concentrations of H2SO4 and NaOH respectively. These setups were allowed to stand for a period of 28 days with a coupon withdrawn from each beaker after every 7 days, processed according to standard procedures before reweighing. The data of the weight loss were recorded and computations of the corrosion penetration rate using the formula; CPR=K∆W/ρA∆t were calculated. Plots of corrosion penetration rate against time, inhibition efficiency against concentration of extract and Langmuir adsorption isotherms were then plotted. The results obtained revealed that the corrosion rate profiles for passivating metals in which there is an initial sharp rise in corrosion rate, followed by a progressive decline as exposure time increased were observed. The inhibition efficiencies of the extract in the various media also showed significant increase in all the media. The Langmuir adsorption isotherms indicated that the adsorption of the molecules of the extract on the metal surface was uniformly distributed over the entire metal surface and also the adsorptive forces were strong enough to cause effective adhesion to prevent further corrosion attack. In conclusion, it was established that Zingiber officinale leaf extracts are good inhibitors of corrosion and that the inhibitive potency of the extract increases with increase in the concentrations of the extract and the corrosion media as exposure time increased, with the extract showing better inhibition characteristics in NaOH than H2SO4.

Effect of industrial alkene ozonolysis on atmospheric H2SO4 formation

This study has employed the master chemical mechanism (MCM) to investigate the influence of the ozone oxidation pathways in the atmospheric formation of H2SO4 from short-chain olefins in industrialized areas. In-situ H2SO4 formation data were obtained using a high-resolution chemical ionization time-of-flight mass spectrometer, and the simulated H2SO4 concentrations calculated using updated parameters for the MCM model exhibited good agreement with observations. In the simulation analysis of different reaction pathways involved in H2SO4 formation, hydroxyl radicals were found to dominate H2SO4 production during the daytime, while olefin ozone oxidation contributed up to 65% of total H2SO4 production during the night-time. A sensitivity analysis of the H2SO4 production parameters has revealed a high sensitivity to changes in sulfur dioxide, and a relatively high sensitivity to olefins with fast ozonolysis reaction rates and bimolecular reaction rates of resulting stabilized Criegee Intermediates. A high relative humidity promotes daytime H2SO4 formation, but has an inhibiting effect during the night-time due to the different dominant reaction pathways.

Mechanism and kinetic study of rare earth extraction from allanite by direct acid leaching

Direct acid leaching was conducted to recover rare earth elements (REEs) from an allanite ore assaying 9,861.4 ppm of REEs. The effects of operational variables, such as acid type, sulfuric acid (H2SO4) concentration, temperature, solid/liquid (S/L) ratio, and particle size, on the total REE (TREE) recovery were systematically examined. The REE mineralogy and the involved leaching mechanisms were investigated by electron microscopy characterization and residue re-leaching tests. It was found that approximately 80% of REEs was extracted using 1 M H2SO4 at 75 °C for 2 h. Increasing temperature significantly improved the REE extraction, while changing acid type, H2SO4 concentration, and particle size did not cause noticeable influences on the extraction efficiency. The low fraction of Si leached (<4.0%) suggested that the extraction did not significantly alter the crystal structure of the dominant minerals. The leaching kinetics of REEs was fast within the first 10 min, followed by a much slower stage (i.e., 10–120 min). The kinetic modeling results showed that both stages were likely controlled by both chemical reaction and diffusion, implying a mixed control mechanism. The activation energies for REE leaching using 1 M H2SO4 were determined to be 20.3 kJ/mol for the 0-10 min stage and 10.8 kJ/mol for the 10-120 min stage. The residue re-leaching results suggested that the initial fast leaching kinetics was caused by the preferential dissolution of metamict allanite, while the following slower leaching stage was due to the dissolution of difficult-to-leach REE-bearing minerals, such as well-crystallized allanite.

Efficient processing rare earth polishing powder solid waste by thiourea-H2SO4 system: Process optimization and thiourea decomposition effect

Utilization of waste rare earth polishing powder (REPPW) is significant for protecting the environment and enhancing strategical rare earth resource recycling. However, efficient extraction of rare earth oxides (REOs) from REPPW in an environmental-friendly and low-cost manner remains a great challenge in this area. In this study, a reductive H2SO4 leaching process using the low toxicity thiourea as the reducing agent is proposed to extract rare earth elements (REEs) from the REPPW. Under the optimal conditions of thiourea concentration of 0.078 mol·L−1, H2SO4 concentration of 1.50 mol·L−1, temperature of 353 K, and liquid-to-solid ratio of 15 mL·g−1, the leaching ratios of Ce and La reached 93.8% and 93.2%, respectively. Moreover, the thiourea behavior during the REPPW leaching process was studied. The results indicates that the thiourea not only acted as reductant but also tended to decompose in a CS(NH2)2→(SCN2H3)2→S route. The decomposed thiourea amounted to 18.07% of its original dosage, which formed precipitation during aging of the leachate. However, it influenced neither Ce/La concentration in the leachate nor purity of the product. The recycled REOs with purities higher than 93% were prepared from the leachate by oxalate precipitation-oxidation roasting, showing no relation with the aging time.

Corncob-Derived Sulfonated Magnetic Solid Catalyst Synthesis as Heterogeneous Catalyst in The Esterification of Waste Cooking Oil and Bibliometric Analysis

A corncob-derived magnetic solid acid catalyst was synthesized through the sulfonation method and an impregnation process, respectively. In the sulfonation process, the concentrated H2SO4 was utilized as an activation agent to obtain acidic properties. The solution of ferric sulphate-ferrous sulphate was utilized for impregnation to generate the magnetic behaviour of the material. The prepared magnetic acid solid catalyst had a high saturation magnetisation value of 16.48 emu/g and a total acidity of 1.43 mmol/g. The performance of the catalyst was evaluated in the esterification reaction of waste cooking oil. The best result presented 86.12% FFA conversion under reaction conditions of 5% catalyst loading and a 1:15 oil-to-methanol molar ratio at 60oC for 4 h. The catalyst was separated magnetically from the reaction solution and exhibited a good reusability with 61% remaining active after 5 consecutive cycles of reaction. This study resulted in a promising method to obtain magnetic-sulfonated carbon-based catalyst from corncob residue, and it is economical potentially and environmentally friendly for the esterification of low-quality feedstock for biodiesel production.