Increase In HF Concentration Research Articles

Effects of thermal disposal of oil-water residues on operation heating plant boiler

Industrial zones are often the source of specifically polluted waters, depending on type of industry. These waters are usually not in the limits for outtake into public sewers and it is necessary to dispose them externally or build-up technologies for their pre-treatment. The article is about the real case of an industrial. A separate sewage network is built for oily water. Solid impurities are separated from the oily water and oily water is treated by evaporation technology. The evaporator produces two types of products - condensate and concentrate. Condensate is a water with high organic content, and it is necessary to purified it by a chemical treatment plant in next step. Evaporator technology remove from water the most of metal content and significantly decrease organic and water content in concentrate. Concentrate is oily stream with lower water content. Water content in concentrate decreased in next technology steps to ca. 40-50 %. The final product is certificated technological fuel, and its final elimination is in the heating plant fluidized bed boiler. The article is detailed focused on the influence of concentrate combustion. A local temperature increases around 10-15 °C extending across the combustion chamber was confirmed. There was also an increase in HF concentration in flue gas. CO concentrations were higher in the first moments of combustion, but the control system responded by increasing the combustion air supply, and CO concentrations were finally lower at the level of 3 mg·m-3. During combustion, there was no provable increase in concentrations of NOX or other substances, and the co-combustion did not have a significant effect on the operation of the boiler, so this way was evaluated like technically and economically effective.

Study on dissolution mechanism of mineral/liquid interface during HF acid leaching niobium-tantalum ore

The aim of the research work was to study the specific process of hydrofluoric acid decomposing (Fe,Mn)(Nb,Ta)2O6 mineral structure. In this paper, X-ray photoelectron spectroscopy (XPS), density functional theory (DFT) calculations combining with leaching experiments were used to study the atoms bonding, charge transfer, and energy changes on the mineral/liquid interface and selective dissolution mechanism during the process of leaching (Fe,Mn)(Nb,Ta)2O6 mineral with hydrofluoric acid. DFT calculation results showed that Ta was more easily dissolved than Nb, and Mn was more easily dissolved than Fe. The core energy level spectrum and XPS detection revealed that Nb and Ta at the same site were preferentially leached from the mineral than Fe and Mn at the same site. Therefore, the order of metal dissolution in mineral from difficult to easy was Fe, Mn, Nb, and Ta. ICP results combined with XPS quantitative analysis showed that with the increase of HF concentration, the leaching rate of Nb and Ta increased, and the bridging F existing on the mineral surface would be converted into terminal F, thereby prompting more metal atoms to dissolve away from the mineral surface. Therefore, the dissolution of metals was more likely to occur at higher concentrations of F−.

Microstructural evolution and magnetic properties of Co-rich Hf-Co alloys

Hf – Co binary alloys with different Hafnium concentration ranging from 12.5 to 20.5 at% have been prepared and characterized for structural attributes and magnetic properties. The alloy with 12.5 at% Hf exhibits a large volume fraction of soft magnetic α-Co(Hf), which impairs the hard magnetic properties. With increase of Hf concentration, the volume fraction of soft magnetic phase decreases with increase in fraction of HfCo7, leading to improvement of hard magnetic properties. The HfCo7 phase is found to be responsible for permanent magnetic behavior of the Co-rich Hf-Co binary alloys. The microstructural evolution has been unraveled through the understanding of solidification sequence and correlated with observed magnetic properties.

Structural, electronic, vibrational, and thermodynamic properties of : A first-principles-based study☆☆Project supported by the National Natural Science Foundation of China (Grant No. 21573200).

The physical properties including structural, electronic, vibrational and thermodynamic properties of Zr1−xHfxCo (x is the concentration of constituent element Hf, and changes from 0 to 1) are investigated in terms of the ABINIT program. The results reveal that all of Zr1−xHfxCo have similar physical properties. When Hf concentration x gradually increases from 0.0 to 1.0, the lattice constant decreases from 3.217 Å to 3.195 Å very slowly. The calculated density of states (DOS) indicates that the metallic nature is enhanced and the electrical conductivity turns better with the increase of Hf. Moreover, as Hf concentration increases from 0 to 1, the Fermi energy gradually increases from −6.96 eV to −6.21 eV, and the electronic density of states at the Fermi level decreases from 2.795 electrons/eV f.u. down to 2.594 electrons/eV f.u., both of which imply the decrease of chemical stability. The calculated vibrational properties show that the increase of Hf concentration from 0 to 1 causes the maximum vibrational frequency to decrease gradually from about 223 cm−1 to 186 cm−1, which suggests a lower dispersion gradient and lower phonon group velocities for these modes. Finally, the phonon related thermodynamic properties are obtained and discussed.

New Zr–Hf Geothermometer for Magmatic Zircons

A geothermometer equation $$T = \frac{{1531}} {{\ln K_d + 0.883}}$$ , where $$K_{\dot d} = \frac{{X_{Zr}^S X_{Hf}^m }} {{X_{Zr}^m X_{Hf}^s }}$$ [X is the concentration (in ppm) of component i in phase j] is the Zr and Hf distribution coefficient between melt and zircon, and T is temperature in K, was derived by thermodynamic processing of literature experimental data on Zr and Hf distribution between acid melts (m) and zircon (s) and on the solubility of zircon and hafnon in the melts with variable silica content. In calculations with this equations, we assumed the Zr concentration in zircon to be constant: 480000 ppm. It is shown that the commonly observed increase in Hf concentration from the cores to margins of magmatic zircon crystals is caused by the fractional crystallization of zircon. For differentiated acid magmatic series, the initial crystallization temperature of zircon in the least silicic cumulates should be evaluated using the cores of large zircon grains with the highest Zr/Hf ratio. Application of the geothermometer for mafic and intermediate rocks may be hampered due to simultaneous crystallization of zircon with some other ore and mafic minerals relatively enriched in Zr and Hf. The newly derived geothermometer has some advantages over other indicators of the crystallization temperature of magmatic zircon based on the zircon saturation index (Watson and Harrison, 1983; Boehnke et al., 2013) and on Ti concentration in this mineral (Ferry and Watson, 2007) as it does not depend on the major-oxide melt composition and on the accuracy of the estimated SiO2 and TiO2 activities in the melts. Calculations of the Zr and Hf fractionation trends in the course of zircon crystallization in granitoid melts allow one to evaluate the temperature at which more evolved melt portions were segregated.

Effect of Fluoride Concentration on Niobium Anodic Dissolution

Niobium is one of the valve metals [1] and, when exposed to air or water, it readily forms a chemically inert and mechanically strong oxide layer [2]. Nb is used as a popular alloying element in carbon and steel industry [3] due to its high corrosion resistance. Nb is also used for fabricating super conducting radio frequency cavities [2] used in particle accelerators which demand very smooth surfaces. The passive layer on Nb is stable in most aqueous media. However, it can be attacked by hydrofluoric acid [4, 5] or strong alkali [6]. Nb anodic dissolution in HF shows clear active and passive regions, followed by an increase and a large current plateau signifying transpassive dissolution [5]. The polarization and EIS data of Nb in warm alkali and acidic fluoride media was analyzed in the framework of surface charge approach combined with interfacial reactions [4]. The species that are reported to be formed in aqueous HF solutions are HF, H+, F-, HF2 - and H2F3 - [7] and the dissociation of HF involves the following reactions. HF ↔ H+ + F-, HF + F-↔ HF2 -, along with HF + HF2 - ↔ H2F3 - The corresponding equilibrium constants are respectively, 6.84 × 10-4, 5 and 0.58 [7]. In this study, we present the experimental results of anodic dissolution of Nb in different concentrations of HF and propose a kinetic model to match the observed results. All the experiments were carried out using Nb rotating disc electrode (RDE) in a standard three-electrode cell with Ag/AgCl (3.5 M KCl) as reference and Pt wire as counter electrode. HF concentration was changed between 50 mM and 1000 mM. In order to reduce the solution resistance effects, a supporting electrolyte (1 M Na2SO4) was used. The electrode rotational speed was maintained at 900 rpm. Before each experiment, the electrode was mechanically polished with 400, 800 and 1200 grade emery paper followed by ultra-sonication in ethanol and DI water. Potentiodynamic polarization data were acquired by sweeping the potential from open circuit potential (OCP) to 1 V above OCP, at 2 mV/s scan rate. Figure 1A shows the typical anodic behaviour of Nb measured as described above in different concentrations of HF used. The peak current values shift towards right with increase in HF concentration. The potential at which the peak occurs also shifts to more anodic values at higher HF concentration. The concentration of all the constituent species increases with increase in nominal HF concentration [8]. Therefore, few experiments were carried out adding H2SO4and KF to aqueous HF solutions to isolate the effect of different species on the dissolution (Fig 1B). Reaction mechanism analysis was employed to understand the dissolution mechanism of Nb in HF. A four step mechanism involving two dissolution steps, chemical and electrochemical was proposed to explain the results. The model captures all the major characteristics of the polarization curves.

Studies on Zirconium Dissolution in Hydrofluoric Acid

Passive films and their stability are getting increased attention in the field of electrochemistry due to a number of applications, [1, 2]. Valve metals such as Ti, Zr, Nb and Ta form insulating or semiconducting oxide films under anodically polarization conditions [2]. Zirconium and its alloys are widely used in nuclear and chemical industries [3] due to its high corrosion resistance even in aggressive media. This resistance is due to the formation of a stable, protective passive ZrO2 film [4]. Although ZrO2 is passive, it can be attacked by halide ions. Previously, Zr corrosion in the presence of halide ions such as Cl-, F-, I-, Br- has been investigated [4]. In particular, the anodic polarization of Zirconium in fluorinated concentrated nitric acid medium was studied and a seven step model with three intermediate species has been proposed to explain the results [5].However, a later study proposed that the mechanism of Zr dissolution in HF is altered by HNO3 and that the nitric acid is simultaneously reduced along with Zr dissolution [6]. Hence these results may not be directly applicable to nitric acid free systems. HF being a weak acid dissociates to a limited extent, and can form multiple species as given below in the three equilibrium reactions. HF ↔ H++F- HF+F-↔HF2 - HF+HF2 -↔H2F3 - The equilibrium constants respectively are K1 = 6.84 × 10-4, K2 = 5, and K3 = 0.58 [7]. In this work, the effect of varying the concentration of HF on Zr dissolution was investigated. A rotating disk Zr electrode was used in a conventional three electrode cell with Ag/AgCl (in 3.5 M KCl) and Pt wire as reference and counter electrode respectively. HF concentration was varied from 5 mM to 200 mM. In all the experiments, 0.1 M Na2SO4was used as the supporting electrolyte. The electrode was rotated at 900 rpm to minimize the mass transport limitations. Potentiodynamic polarization experiments were conducted starting at 250 mV below open circuit potential and ending at 2500 mV above OCP on the anodic side. The scan rate was maintained at 2 mV/s in solutions with HF concentration of 50 mM or less. At 100 mM and 200 mM HF, the scan rates were increased to 20 mV/s since a black film formed on the electrode at lower scan rates and caused a sudden change in current values. The results are presented in Fig. A. The anodic polarization curves (Fig. A) show that the active and passive regimes are clearly visible in all the HF concentrations studied here. The potential the current in the cathodic and anodic regime (restricted to active region) are presented in Fig. B, with the current in logarithmic scale. The corrosion currents were estimated by extrapolating the cathodic and anodic branches. Fig. A shows that the current increases in the active region, attains a peak value and there is a slight decrease in the current at higher potentials. Thepeak current value increases with an increase in HF concentration. At higher concentration, the peak potential location shifts to more positive values at higher HF concentrations. In each solution, the remaining amount of HF, H+, F-, HF2 - were calculated using the equilibrium constants and individual correlation to the corrosion current and peak current were evaluated. The corrosion and peak current values were correlated with the concentration of individual species occurring at equilibrium. A few experiments were also conducted in the presence of H2SO4 or KF to help identify the species involved in the dissolution. The corresponding changes in the peak current values and corrosion current estimates were also analysed. The results indicate the peak current can be expressed as proportional to [H+]α and [F-]βwhere α is less than 1 and β is greater than 1, indicating that undissociated HF plays an important role in the reaction mechanism.

Anodic Dissolution of Titanium in Acidic Fluoride Media - Effect of Varying Fluoride Concentration

Ti and Ti alloys are well known for their excellent chemical stability and biocompatibility. Ti metal shows high corrosion resistance due to the presence of protective native oxide film. Although TiO2 layer is stable and protective in action, it can be dissolved in HF [1-3]. Anodic dissolution of Ti in acidic fluoride medium shows three distinct regions, viz. active, passive and transpassive [1,2]. The anodic current increases with potential in the active region and it decreases with potential in the passive region. The current increases slowly with potential in the transpassive region. Transpassive dissolution has been extensively studied [1,3]. Using potentiodynamic polarization and electrochemical impedance spectroscopic data, the anodic dissolution of Ti in 0.1 M HF in the active and passive regime has been investigated recently and a four step mechanism has been proposed [2]. HF is a weak acid and is known to form various species in solution, as described in the equation below. HF ↔ H++F- HF+F-↔HF2 - HF+HF2 -↔H2F3 - The equilibrium constants for the above reactions are K1 = 6.84 × 10-4, K2 = 5 and K3 =0.58 respectively [4]. Although a four step mechanism has been proposed to explain the anodic dissolution of Ti in HF[2], the influence of HF concentration has not been incorporated in the mechanism yet. It is important to quantify each species present in solution and their influence on individual steps of the reaction. In this work, the effect of varying the concentration of HF on Ti dissolution was investigated. A rotating disk Ti electrode was used in a three electrode cell with Ag/AgCl (in 3.5 M KCl) as reference and Pt wire as counter electrode. HF concentration was varied from 25 mM to 1000 mM. In all the experiments, 1 M Na2SO4 was used as the supporting electrolyte. The electrode was rotated at 900 rpm. Potentiodynamic polarization experiments were conducted at a scan rate of 20 mV/s, starting at -250 mV below open circuit potential and ending in the transpassive regime on the anodic side. A few experiments were also conducted at a scan rate of 2 mV/s, in solutions of 50 mM, 500 mM and 1000 mM HF concentrations, and the current response was the same at both 2 mV/s and 20 mV/s scan rates. The results are presented in Fig. A. The anodic polarization curves (Fig. A) show that the active and passive regimes are clearly visible in all the HF concentrations studied here. The inset shows the results at lower concentrations, expanded for clarity. The peak current value increases with an increase in HF concentration. The location of peak potential also shifts to more anodic values at higher HF concentration. The potential and logarithm of the absolute value of current in the cathodic and anodic regime (active region only) are presented in Fig. B. The cathodic and anodic branches of potentiodynamic polarization were extrapolated to estimate the corrosion current. The estimated corrosion current values and the peak current values near active-passive transition were correlated with nominal HF concentration and as well as with the concentration of individual species occurring at equilibrium. In order to identify the effect of individual species unambiguously, a few experiments were also run by adding H2SO4 or KF to the experimental solutions (results not shown here). The corresponding changes in the peak current values and corrosion current estimates were also analysed. The results indicate the corrosion current and peak current can be expressed as proportional to [H+]α and [F-]β , where α is 0.72 and β is 0.92. This suggests that undissociated HF is likely to play a major role in Ti dissolution.

Electrochemical formation of GaAs honeycomb structure using a fluoride-containing (NH4)2SO4 solution

GaAs substrates were anodized in the (NH4)2SO4 electrolyte with various fluoride concentrations. Scanning electron microscope (SEM) observation showed that highly regular honeycomb hollows were formed on the substrates anodized in the (NH4)2SO4 electrolyte with a small amount of HF concentration. The regularity of hollows decreased with the increase of HF concentration. The average diameter of hollows increased with increasing anodizing voltage. The regularity of hollow diameters increased with the increase of anodizing time, irrespective of the anodizing voltage. Cross-sectional SEM image showed that the average depth of regular hollows was about 5nm. In addition to the peak in the region of fundamental adsorption of GaAs with the peak wavelength at about 870nm, photoluminescence spectra of samples anodized in the (NH4)2SO4 electrolyte with HF concentration of 0.5ml showed several peaks at about 610, 635, 670 and 720nm.

Dependence of Performance of Si Nanowire Solar Cells on Geometry of the Nanowires

The dependence of performance of silicon nanowires (SiNWs) solar cells on the growth condition of the SiNWs has been described. Metal-assisted electroless etching (MAE) technique has been used to grow SiNWs array. Different concentration of aqueous solution containing AgNO3 and HF for Ag deposition is used. The diameter and density of SiNWs are found to be dependent on concentration of solution used for Ag deposition. The diameter and density of SiNWs have been used to calculate the filling ratio of the SINWs arrays. The filling ratio is increased with increase in AgNO3 concentration, whereas it is decreased with increase in HF concentration. The minimum reflectance value achieved is ~1% for SiNWs of length of ~1.2 μm in the wavelength range of 300–1000 nm. The performance and diode parameters strongly depend on the geometry of SiNWs. The maximum short circuit current density achieved is 35.6 mA/cm2. The conversion efficiency of solar cell is 9.73% for SiNWs with length, diameter, and wire density of ~1.2 μm, ~75 nm, and 90 μm−2, respectively.

Optical properties of porous silicon on an insulator layer

N-type silicon wafers, consisting of 280 μm upper and 20 μm thick lower layers, were electrochemically etched in a hydrofluoric acid (HF) ethanol solution. The resistivity of the upper layer was 0.015 Ω cm, while the lower layer was a much worse conductor with a resistivity of 2 Ω cm. Porous silicon (PS) samples were produced by etching the rough (upper) side of single-side polished wafers at a constant current density. The process of etching was monitored at different HF concentrations. The samples were investigated by Raman spectroscopy, photoluminescence (PL) and scanning electron microscopy (SEM). Due to the roughness of the unpolished surface, different surface orientations were exposed to electrochemical etching, which resulted in different etching speed and consequently a different morphology (plateaus, valleys) produced by etching. The porous plateaus showed the most intensive PL observable, even by optical microscope. PL spectra exhibited a decrease of peak intensity and the blue shift of maximum with an increase of HF concentration. The presence of nanometer-size Si structures was confirmed by the broadening and red shift of the transversal optical (TO) phonon band in the Raman spectra. The quantum confinement model was used to determine the average size of these structures. SEM images showed pores of different morphology and several nanometers in diameter. The largest pores and thinnest walls were obtained when etched with the lowest HF concentration.

Influence of hydrofluoric acid on extraction of thorium using a commercially available extraction chromatographic resin

The dependence of Th recovery on hydrofluoric acid (HF) concentration in nitric acid (HNO 3) solutions (1–5 mol/dm 3) containing 1 × 10 −6 mol/dm 3 of Th and various concentrations of HF and the elution behavior were studied using a commercially available UTEVA (for uranium and tetra valent actinide) resin column. Thorium recovery decreased with an increase in HF concentration in the sample solutions. The concentration of HF at which Th recovery started to decrease was ∼1 × 10 −4 mol/dm 3 in 1 mol/dm 3 HNO 3 solution, ∼1 × 10 −3 mol/dm 3 in 3 mol/dm 3 HNO 3 solution, and ∼1 × 10 −2 mol/dm 3 in 5 mol/dm 3 HNO 3 solution. When Al(NO 3) 3 (0.2 mol/dm 3) or Fe(NO 3) 3 (0.6 mol/dm 3) was added as a masking agent for F − to the Th solution containing 1 × 10 −1 mol/dm 3 HF and 1 mol/dm 3 HNO 3, Th recovery improved from 1.4 ± 0.3% to 95 ± 5% or 93 ± 3%. Effective extraction of Th using UTEVA resin was achieved by selecting the concentration of HNO 3 and/or adding masking agents such as Al(NO 3) 3 according to the concentration of HF in the sample solution.

Formation of porous TiO x biomaterials in H 3PO 4 electrolytes

In this work, formation of porous TiO x layers and theirs corrosion behavior were studied. Application of H 3PO 4 electrolytes results in porous TiO x formation. The process is enhanced by small amount of HF content in the electrolyte. The HF results in higher current density, enhancing dissolution. Small 0.5% HF concentration results in nanopores formation, with pore diameter of about 45 nm. Increase of HF concentration up to 10% results in pores with average diameter of about 5.2 μm. An increase of etching time results in larger pore diameter, but between large 2–5 μm diameter pores smallest ones were observed with diameter below 200 nm. In the initial etching process a remnants of the flat surface are presents with initial cracks in the surface, indicating places for growth of the pores. The TiO x layers can be used as a biomaterial. The corrosion behavior of the layer investigated in Ringer’s solution, revealed an excellent corrosion resistance, with respect to pure Ti.

Effect of anodization conditions on the synthesis of TiO2 nanopores

ABSTRACTNanoporous structures play a promising role in the development of nanomechanical, nanoelectrical and biosensing devices. In addition, nanopores can be utilized as chemical and gas sensors. TiO2 is a semiconductor material, which can have a wide range of applications in nanopore-based sensors. In this study, TiO2 nanopores were prepared by electrochemical anodization. Titanium was used as the anode, while platinum was used as the cathode in an electrochemical cell filled with a hydrofluoric acid electrolyte solution. During the preparation process, titanium was converted to its oxide form. Nanostructures were synthesized under varying physical conditions, including HF concentrations of 0.5-10% and anodization times of 5-30 minutes. The resulting nanopore structures were characterized by scanning electron microscopy (SEM). With a progressive increase in HF concentration (from 0.5% to 10%), the diameter of the nanopores decreased, from approximately 100 nm in diameter to 50 nm. The nanopores showed a transformation from tube-like structures to pore networks with increased HF concentration or anodization time. The results show that the dimensions and morphology of the nanopores can be controlled by alteration of the anodization conditions.

Concentration Effect of HF on Energy Band Diagram for n-Si(100)/HF Photoelectrochemical Etching System

The aim of this work was to build a thermodynamic energy band diagram for the system of n-type Si(100)/HF that is in dynamic equilibrium at the interface. The construction concept was based on the shift of energy levels such as Fermi energy conduction band energy and valence band energy before and after the contact of silicon with HF solutions. Through measurements of the open-circuit potential and flatband voltage the energy band diagram for the Si/HF system was established. This diagram was useful in estimating the activation energy for the photoelectrochemical etching system. The kinetic study demonstrated that the etching rate of the silicon increases to a maximum with an increase in HF concentration from 0.5 to 2.0 M, then decreases with further increase of the HF concentration. Based on the energy band diagram established and the electrochemical kinetic data measured, we discuss in detail the etching rate of the n-Si/HF and make clear the concentration effect of HF on the photoelectrochemical reaction. © 2005 The Electrochemical Society. All rights reserved.

Application of fluorine-containing gases for cleaning CVD systems in consideration of global environmental problems—2—

Amorphous silicon nitride and amorphous silicon are deposited by plasma CVD in the TFT (Thin Film Transistor) fabrication. The characteristics of the etching reaction of a- Si:H and a- SiNx:H with ClF3 and the mixed gas of ClF3 and HF which have no direct global warming potential (GWP) were studied. Amorphous silicon was rapidly etched by ClF3 even at room temperature and the etch rate depended on the partial pressure of ClF3 below 72 Torr. On the other hand, the etch rate of amorphous silicon nitride varied slowly. Further, the etch rate of amorphous silicon nitride could not be accelerated by increasing the partial pressure of ClF3 beyond 14 Torr.We found that the addition of HF to ClF3 accelerated greatly the etch rate of amorphous silicon nitride. However, an increase in HF concentration of more than 1vol% was not be effective on the etch rate of amorphous silicon nitride at 70°C, under 700 Torr, when ClF3 concentration was 10vol%.

Fluorine-containing species on the hydrofluoric acid etched silicon single-crystal surface

The chemical structure and property of fluorine-containing species on the hydrofluoric acid (HF) etched Si surface was examined by use of x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The fluorine content on the surface was found to increase with increase of HF concentration. A silicon surface etched by 50% HF has fluorine of 2.6×1014 atoms/cm2 as Si–F. Most of the Si–F bondings are rapidly hydrolyzed to Si–OH by rinsing the wafer in water. Thus prepared Si–OH groups are found to be useful as active sites for chemical modification of the bare silicon single-crystal surface. The Si–F was observed not to influence the oxidation rate of HF etched silicon surface.