Search

Ce-doped α-Fe2O3 nanoparticles were successfully synthesized by hydrothermal method at different pH. The relationship between the pH of the solution and the morphology, structure and electrochemical stability of the prepared Ce-doped α-Fe2O3 nanoparticles was investigated by X-ray diffraction, transmission electron microscopy, scanning electron microscope, Fourier transform infrared, X-ray photoelectron spectroscopy, electrochemical methods and saltwater immersion experiment. The results showed that the Ce-doped α-Fe2O3 nanoparticles prepared at pH = 4 and pH = 6 had surface defects structure, and the Ce-doped α-Fe2O3 nanoparticles prepared at pH = 8 had adhesion structures, which were CeO2 nanoparticles adhered to the α-Fe2O3 nanoparticles’ surface. The fact that Ce ions could be readily doped into the α-Fe2O3 lattice, causing lattice distortion and increasing the binding energy of Fe3+ in the lattice, thereby enhancing the stability of Fe–O bonds correspondingly. At the same time, the surface defect structure is produced, which has the effect of promoting the compactness of the coating. The surface defects structure of α-Fe2O3 has stronger electrochemical stability than the adhesion structure of α-Fe2O3 and Bayer α-Fe2O3. It was found that waterborne acrylic coatings prepared from of α-Fe2O3 with surface defects structure had a stronger hindering effect on the diffusion of charged ions.

H2SO4-H2O2 mixtures are a promising and environmentally friendly passivation medium for the stainless-steel pickling process. The corrosion behavior of stainless steel is highly dependent on the kinetics of passive film growth. Long-term electrochemical measurements, including polarization resistance, open circuit potential (OCP), and electrochemical impedance spectroscopy (EIS) measurements were performed to investigate the evolution of the passive state of 304 stainless steel. According to the OCP results, an active-passive transition takes place in 10 ks in 0.5 M H2SO4 solution containing 0.005 M to 0.3 M H2O2. Polarization resistance results indicate that the passive film thickness keeps growing after OCP stabilization in the presence of H2O2. Electrochemical impedance spectroscopy (EIS) results confirmed that the growth of the passive film in H2SO4-H2O2 solutions takes about 9 h. Additionally, according to the Point Defect Model (PDM) and Mott–Schottky analysis, the semiconductor properties of the passive film on 304 stainless steel in H2SO4-H2O2 solution were studied. The results indicate that the passive film is an n-type semiconductor. The donor density is in the range of 1.6 × 10−21 cm−3 to 24 and decreases exponentially with increasing film formation potential (this potential coincides with the final OCP in the corresponding H2SO4-H2O2 solutions). By postulating that most donors are oxygen vacancies, the point defect properties including diffusivity and electrical field strength are obtained.

Usually, boric acid obtained from the high‑magnesium borate minerals by sulfuric acid leaching contains many impurities and shows relatively low yields. This happens due to Mg2+ and H3BO3 in the leachates, which are challenging to separate effectively using H2SO4 as a leaching agent. In this work, an innovative approach to solving the disturbance of Mg2+ on solution purification was proposed. It was found that more than 90% Mg2+ could be removed in the form of the MgSO4·H2O crystalline phase with a hydrothermal process. The ratio of H3BO3 to Mg2+ in the aqueous solution was significantly increased in primary separation. The amount of H3BO3 carried in the crystalline phase was

Oxide layer formed on stainless steel surface can result in a decrease in surface quality. Thus, it must be removed. In this paper, we investigated the effect of different pickling processes on rem...

The main purpose of the present work was to study the passivity of stainless steel in sulphuric acid under chemical oxidation with H2O2 solution. Potentiodynamic polarisation and open circuit poten...

The acid-base behavior on nanoparticles’ surface played an important role in chemical reactions related to catalysis, adsorption, and corrosion resistance. Exploring the characteristics of ≡FeOH on the surface of nanoparticles was helpful to study the acid-base behavior of the surface of nanoparticles. It provided a basis for the development of new surface active materials. In this study, cerium oxide/iron oxide nanocomposites were synthesized by physical blending method (PBM-CION) and hydrothermal method (HM-CION). HM-CION had lattice distortion, while PBM-CION was only physical adsorption, and the crystal structure had not changed significantly. The band gap of HM-CION was significantly reduced to 2.16 eV and it’s ≡FeOH concentration was 1.9 times and 1.3 times of that in PBM-CION and Bayer iron oxide. Therefore, it had a strong acid-base buffering capacity in the solution, so HM-CION had potential application value in catalysis, adsorption, and anticorrosive coating. HM-CION mainly undergone ≡FeOH protonation reaction, and the equilibrium constants of ≡FeOH protonation and deprotonation reactions are − 4.40 and − 8.22, respectively.

A new leaching process for preparing chromium oxide from chromite was studied in this paper, using Cr(VI) extracted from the remediation treatment of chromite ore processing residue (COPR) as an oxidant. Thus, the “detoxification” of deposited COPR could be achieved and no new Cr(VI)-bearing substances would be generated in preparation process of chromium oxide. Thermodynamic analysis was applied and several tests were conducted. It was found that a chromite with higher Fe(II) content showed lower stability in an oxidizing solution. A general decomposition mechanism of chromite is proposed and suggests that the oxidation of ions in spinel plays a significant role in decomposing chromite. Furthermore, the extraction process of chromium from leachate was investigated. Results demonstrated that iron, magnesium and aluminum could be effectively removed via oxalic acid treatment and subsequent pH adjustment, with chromium oxide as the final product.

Plenty of flue gas desulfurization (FGD) gypsum generated from coal-fired power plants for sulfur dioxide sequestration caused many environmental issues. Preparing calcium sulfate whisker (CSW) from FGD gypsum by hydrothermal synthesis is considered to be a promising approach to solve this troublesome problem and utilize calcium sulfate in a high-value-added way. The effects of particle size of FGD gypsum, slurry concentration, and additives on CSW were investigated in this work. The results indicated that fine particle size of FGD gypsum and moderately high slurry concentration were beneficial for crystal nucleation and growth. Three additives of magnesium chloride, citric acid, and sodium dodecyl benzene sulfonate (SDBS) were employed in this study. It was found that mean length and aspect ratio of CSW were both decreased by the usage of magnesium chloride, while a small quantity of citric acid or SDBS could improve the CSW morphology. When multi-additives of citric acid-SDBS were employed, the mean length and aspect ratio increased more than 20%. Moreover, surface morphology of CSW went better, and the particle size and crystal shape became more uniform.

Ce-doped nano-&alpha, Fe2O3 was successfully synthesized via the hydrothermal method. The properties of the prepared particles were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and electrochemical methods. It was found that the Ce element can be doped into the &alpha, Fe2O3 lattice resulting in lattice distortion, which can refine the grain and improve the crystal surface&rsquo, s integrity significantly. In addition, doping of Ce element can shorten the Fe&ndash, O bond length in the &alpha, Fe2O3 crystal, cause a blue shift of the stretching vibration band, enhance binding energy of Fe&ndash, O and the chemical stability of the &alpha, Fe2O3 crystal.

To shorten the time required for the pickling process and to enhance the quality of ferritic stainless steel plates, the effects of oxidants including hydrogen peroxide (H2O2), potassium permanganate (KMnO4), and potassium chlorate (KClO3) on the pickling behavior in HCl-based electrolyte as well as the surface quality of hot-rolled and blasted 430 stainless steel (430-SS) were studied. Experiments were conducted using mass-loss tests, microstructure analyses, potcntiodynamic polarization curves, and electrochemical impedance spectroscopy measurements. The results showed that the addition of oxidants substantially accelerated the pickling process of 430-SS by enhancing the cathodic reaction rate and reducing the charge transfer resistance. In electrolytes comprising 5 — 8 mass% HCl at a temperature of 40 — 60 °C and at the same concentration within the range from 0 to 2 mass%, H2O2 was demonstrated to be superior to KMnO4 and KClO3 in accelerating the pickling process. The surface quality of 430-SS pickled in the presence of H2O2 was better than those of specimens pickled in the presence of KMnO4 and KClO3 when the removal of the oxide layer, intergranular corrosion, and surface roughness were collectively considered. When 1 mass% H2O2 was added, the mass loss rate of 430-SS was increased by 629% and no residual oxide layer or intergranular corrosion was observed on the surface of the steel; in addition, the roughness was only 1. 7 μm. H2O2 was determined to be a better oxidant than KMnO4 and KCIO.3 when the pickling process, surface quality, solution recycling, and environment protection were considered as a whole.

In order to obtain hydrophobic whisker for preparing polymeric composite product, the calcium sulfate whisker (CSW) prepared from flue gas desulfurization (FGD) gypsum by hydrothermal synthesis was modified by various surfactants, and the effects of some modification conditions on the hydrophobic property of CSW were investigated in this study. Sodium stearate was considered to be a suitable surfactant and its reasonable dosage was 2% of ethanol solvent. Both physical and chemical absorptions were found in the surface modification process, and the later one was suggested to preferentially occur on the CSW surface. Moreover, modifying temperature, modifying duration, and agitation speed were experimentally found to have a remarkable influence on the modification behavior. Active ratio reached 0.845 when the modification process was conducted under reasonable conditions obtained in the current work. Finally, polypropylene sheet products were prepared from modified CSW showing an excellence mechanical property.

Sulfuric acid-based leaching is a promising cleaner method to produce chromium salts, but its feasibility for treating low Fe(II)-chromite still remains to be proven. A Box&ndash, Behnken design (BBD)-based set of experiments for sulfuric acid leaching of low Fe(II)-chromite was utilized in this work for generating an experimental dataset for revealing the functional relationships between the processing parameters and the extraction yields of Cr and Fe. The dependent variables were found to exhibit strong intercorrelations and the models developed on the basis of statistical criteria showed excellent prediction accuracy. The optimum process conditions of leaching treatment were found to be a temperature of 176 &deg, C, a dichromic acid/chromite mass ratio of 0.12, and a sulfuric acid concentration of 81%. Furthermore, the dissolution behavior of chromite in the leaching process and the effect of dichromic acid were experimentally investigated. It was found that the decomposition efficiency was highly dependent on the Fe(II) content of chromite, and that the dichromic acid acted both as an oxidant and a catalyst in the leaching process. On the basis of the results of this study, a novel process for treating low-Fe(II) chromite was proposed.

Raw materials were co-sintered with municipal solid waste incineration (MSWI) fly ash through iron ore sintering to promote the safe treatment and utilization of MSWI fly ash. To assess the feasibility of this co-sintering method, in this study, the effects of the addition of MSWI fly ash on the formation and emission of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) were estimated via iron ore sintering pot experiments. During co-sintering, most of the PCDD/Fs in the added MSWI fly ash were decomposed and transformed into PCDD/Fs associated with iron sintering, and the concentrations of lower- and mid-chlorinated congeners increased. As there was a sufficient chlorine source and the sintering bed permeability was decreased by the addition of MSWI fly ash, the PCDD/F concentration in the exhaust gas increased. The mass emission of PCDD/Fs decreased; however, the emission of toxic PCDD/Fs increased beyond the total emissions from the independent MSW incineration and iron ore sintering processes due to the transformation of PCDD/F congeners. The co-sintering may be an important solution after technological improvements in the flue gas cleaning system and PCDD/F formation inhibition procedures.

The leaching behavior of chromite by sulfuric acid in the presence of oxidant was studied experimentally in this work, where most tests were carried out at 160 °C. The results indicated that magnesium and aluminum were leached out together from the silicate phase and porous amorphous silica was produced as a product layer, while the spinel phase gradually shrank with the duration of leaching. The separation of silicate and spinel phases had an influence both on the extraction yield of chromium and the leaching reaction rate by reducing the resistance presented by the silicon-rich phase. It was found that the erosion along the phase boundary was more severe than in other areas, and it can be speculated that groovy corrosion in the spinel phase occurred and developed at the grain boundary. The precipitation behavior of sulfate during the leaching process was also investigated at different temperatures up to 200 °C, showing that the sulfate precipitated in the silicon-rich phase at higher temperatures, while the main phase of the sulfate was chromium sulfate, coinciding with results reported by other researchers. All considered metal elements in the spinel phase leached out together in the experiments of this study.

The effects of temperature, duration, sulfuric acid concentration and oxidant on the leaching of South African chromite were investigated in this work. It was found that the extraction yield of chromium increased with temperature, limited by a chromium-rich sulfate precipitating from the solution when the temperature became too high, resulting in the decline of chromium recovery. The spinel phase of the ore shrank while the extraction yield of chromium increased with the duration of the leaching, leveling out after 60 min. A porous amorphous silica phase was obtained as the metal elements leached and the spinel phase flaked, creating a resistance for the dissolution of the inner spinel phase. The results also showed that the sulfuric acid concentration and oxidant have a marked influence on the extraction yield of chromium and on the leaching reaction rate, and the best results were obtained for 80%-wt sulfuric acid and an oxidant/chromite (mass) ratio oxidant above 1/10. The extraction yield of chromium was improved by the employment of agitation, and 1800 rad s− 1 was the reasonable speed.

The effect of potassium hypermanganate (KMnO4) as an oxidant on pickling behavior in HCl-based electrolyte and the surface quality of original hot-rolled and blasted 430 stainless steel (430-SS) plate are studied,using weight-loss tests, microstructure analyses (scanning electron microscope and laser scanning microscope), potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) measurements. The results showed significantly accelerated pickling process for 430-SS by adding KMnO4 through enhanced cathodic reaction rate and reduced charge-transfer resistance (Rt). The weight loss rate of 430-SS is 83.6% higher under the condition of 1wt% KMnO4 than that without oxidant, which grows an enormous 2.33 times with KMnO4 content of 2wt%. The 430-SS plate is left as a total clean one without any oxide layers left or any kind of local corrosion, when 1.5wt% KMnO4 is added. KMnO4 can be confirmed as an appropriate oxidant when many factors such as accelerating the pickling process in the process of the pickling, improving the 430-SS’ surface quality, solution recycling possibility and environment protection are considered as a whole. Keywords-Potassium hypermanganate (KMnO4); 430 Stainless steel; HCl-based pickling; Polarization curves; EIS

The sodium salt roasting process was widely used for extracting vanadium due to its high yield rate of vanadium. However, the serious pollution was a problem. The calcium roasting process was environmentally friendly, but the yield rate of vanadium was relatively lower. Focusing on the calcium metavanadate produced in the calcium roasting process of vanadium minerals, the mechanism of the carbonate leaching for calcium metavanadate and its leaching kinetics of calcium metavanadate were studied. With the increase of the leaching agent content, the decrease of the particle size, the increase of the temperature and the increase of the reaction time, the leaching rate of vanadium increased, and the constant of reaction rate increased. In the carbonate leaching process, the calcium carbonate was globular and attached to the surface of calcium metavanadate. In the solution containing bicarbonate radical, lots of cracks formed in the dissolution process. However, the cracks were relatively fewer in the solution containing carbonate. In the present study, the carbonate leaching for calcium metavanadate was controlled by diffusion, the activation energy reached maximum and minimum in the sodium bicarbonate and the sodium carbonate solution, respectively. The activation energy value in the ammonium bicarbonate solution was between those two solutions. The kinetic equations of the carbonate leaching for calcium metavanadate were as follows: 1 − 2/3η − (1 − η)2/3 = 4.39[Na2CO3]0.75/r0 × exp(−2527.06/T)t; 1 − 2/3η − (1 − η)2/3 = 7.89[NaHCO3]0.53/r0 × exp(−2530.67/T)t; 1 − 2/3η − (1 − η)2/3 = 6.78[NH4HCO3]0.69/r0 × exp(−2459.71/T)t.

Because of the acute toxicity of Cr(VI)-bearing substances, the pollution problem caused by chromite process residue has become a worldwide concern. In the view of relevant studies, the technologies based on the alkali treatment cannot fundamentally resolve the pollution problem, because the oxidation of Cr(III) to Cr(VI) is unavoidable during chromite decomposition. In contrast, the oxidation of Cr(III) to Cr(VI) can be controlled by the sulfuric acid treatment of chromite, and the Cr(VI) pollution can be eliminated from the original source of production. Many research studies focusing on the resolutions of the key obstacles hindering the development of the sulfuric acid treatment process have been carried out, and significant progress has been achieved. In this study, a clean hydrometallurgical process without the generation of hexavalent chromium is demonstrated. First, the chromite was decomposed and leached by sulfuric acid solution in the presence of an oxidant. Then, iron was hydrothermally removed from the acid solution as the precipitate of jarosite. Finally, chromium salts were obtained by adjusting the basicity of the solution, separation and drying. With the aim of realizing industrialization, future research emphasis on the development of the sulfuric acid treatment process is proposed in this study.