HF Leaching Research Articles

A review of tantalum resources and its production

Tantalum (Ta) is a technology-critical metal, which has important applications in many strategic emerging industries, such as electronic, and aerospace, and there are no substitute materials for Ta so far. The supply crisis of Ta has kept growing globally over the past two decades. This work presents a comprehensive review of tantalum resources and its production. There is less than 300 kt Ta in the earth's crust, and the production of primary Ta is an energy- and materials-demanding process. Ta can also be recovered from tin smelting slag, lithium ores, end-of-life tantalum products, etc., and they can account for 37% of the total Ta production. The HF leaching−solvent extraction−sodium potassium fluorotantalate reduction process is the most widely used technology in producing Ta. However, long process and environmental issues have limited the sustainable development of this method. Clean production technologies and efficient recycling of Ta should receive continuous attention.

A new and effective method for quartz-feldspar separation for OSL and CRN dating

Quartz is a preferred mineral which is widely used for optically stimulated luminescence (OSL) and cosmogenic radionuclide (CRN) dating methods. Any contamination in the sample may mislead and result in erroneous results for both the dating methods (e.g. OSL or CRN). Therefore it is essential to get pure quartz before the measurements of OSL and CRN. Presence of other minerals can introduce unaccounted luminescence signal in the case of OSL, similarly in the case of CRN unaccounted concentration of 10 Be and 26 Al may be introduced. Therefore, in order to get reliable ages from CRN and OSL, extraction of pure quartz from the sediment becomes necessary. Protocols have been developed to extract pure quartz from the sediment, e.g., the separation of quartz from feldspar and other heavy minerals can be achieved by applying 2-step density separation. This is followed by HF leaching which usually removes the feldspar contamination from the quartz (if any left). However, in some cases, this method is not successful, especially in the case of feldspar rich samples or in the samples with intergrowth of feldspar in quartz crystals. This paper reports a method that is effective in separating quartz and feldspar. A small amount of fine grained iron powder was used to make feldspar as magnetic mineral and it is separated from quartz using isodynamic magnetic separator effectively. Efficacy of the method was tested with different analytical instruments such as scanning electron microscope (SEM), OSL/TL reader and inductively coupled plasma mass spectromete (ICP-MS). The analytical tests showed that the proposed method eliminates feldspar by ~95%.

Insight into structural evolution and detailed non-isothermal kinetic analysis for coal pyrolysis

In this study, the pyrolysis of low-ash coal (SF) and high-ash coal (NM) was investigated using a thermogravimetric analyzer. The semi-pyrolyzed char was collected to analyze its structural characteristics, and different model-fitting approaches were adopted for non-isothermal kinetics. Partial demineralized coal was also prepared to clarify the mineral–organics–gas correlation. The results showed that coal pyrolysis was a two-step sequence in which the aliphatic chain decomposed first (<600 °C), and then the aromatic ring condensed (>600 °C) with the formation of H2. Moreover, the amorphous structure in coal char was generated before the graphitization. HF leaching can promote the generation of H2 during the secondary pyrolysis of high ash coal. Inner minerals can promote graphitization under high-temperature pyrolysis. The nth-order model showed the best fitting performance, and the activation energy (Ea) increased with temperature based on piecewise analysis. Moreover, the Ea distribution became more concentrated after demineralization, especially for high-ash coal. Finally, the coal pyrolysis mechanism was summarized based on the solid structural evolution with consideration of the gas release property and inner minerals effect.

Porosity and nanostructure of silicon oxycarbide derived carbon

The unique nanostructure of polymer derived pseudo-amorphous silicon oxycarbide ceramics, which comprises of SiO2 nanodomains, turbostratic graphite, and mixed bond tetrahedral of Si-O/C, enables processing of nanostructured materials with enhanced specific surface area and nanoporosity. Selective etching of the SiO2 nanodomains in a polysilsesquioxane derived SiOC structure left behind highly porous carbonaceous materials with specific surface area in excess of 800 m2g-1, where the obtained porosity and their distribution are remnant of the SiO2 nanodomains. High resolution transmission electron microscopy clearly identified the nanopores and graphene type layers in the oxycarbide derived carbons. The carbon in the SiOC ceramics was found to be better ordered after HF leaching process. Volume fraction of the micropores in the derived carbon was found to increase with the pyrolysis temperature of the parent silicon oxycarbide.

Interfacial Structure Change and Selective Dissolution of Columbite–(Fe) Mineral during HF Acid Leaching

The goal of the paper is to study the charge transfer and reactions at the columbite-(Fe) (FeNb2O6) mineral surface during the HF leaching process. In this paper, X-ray photoelectron spectroscopy (XPS), leaching experiments, and density functional theory (DFT) calculations were used to study the surface element adsorption, charge distribution, chemical state, and energy changes of the mineral surface during the process of leaching columbite–(Fe) with different concentrations of hydrofluoric acid. The results showed that as the concentration of F atoms was increased during the acid leaching process, the Nb–O bond was more likely to be broken than the Fe–O bond; the amount of charge transferred from Nb atom to F atom (0.78 e–0.94 e/atom) was greater than that from Fe atom to the F atom (0.25 e–0.28 e/atom), so it was determined that compared to Fe atoms, it was easier for the Nb atoms to bind to F. The results of XPS analysis showed that the electron binding energies of Nb5+–O, Fe3+–O, and Fe2+–O bonds on the mineral surface increased sequentially, and the M–O bond broke during the acid leaching process, forming more stable M–F bonds. Therefore, the Nb5+–F bonds were easier to form a stable structure. Combined with the ICP results, it was found that in the filtrate after 5M HF and 10M HF acid leaching minerals, c(Nb)/c(Fe) were 2.69 and 2.95, respectively, and the concentration ratio of Nb to Fe element in the mineral was 2 which was lower than 2.69 and 2.95, confirming the result of DFT calculation and illustrating that Nb atoms in columbite-(Fe) mineral were more soluble than Fe atoms.

Mechanism of enhancing Fe removal from metallurgical grade silicon by metal-assisted chemical leaching

In this study, the mechanism of enhancing Fe removal from metallurgical grade silicon (MG–Si) by the novel metal-assisted chemical leaching (MACL) purification methods have been investigated. The typical impurity phase Si2Fe was selected to study interface reactions between the impurity phase and lixiviant. Based on density functional theory (DFT), the interactions of HF leaching, CuACL and AgACL on Si2Fe were revealed through studying the evolution behaviors of atom geometric structure, density of states and bond population, which showed that the stability of Si2Fe structure weakened with the orders of AgACL > CuACL > HF leaching. It was found that the lengths of Si–Fe bonds and metallicity of Si2Fe were successively increased when Si2Fe interacted with HF leaching, CuACL and AgACL, respectively. Compared with HF leaching, MACL weakened Si–Fe bond populations and increased HFe bond populations, further destroyed the stability of Si2Fe structure. The experimental results showed that the impurity Fe removal rates were 94.30% for HF leaching, 95.75% for CuACL, and 97.27% for AgACL, respectively, which was highly consistent with the theoretical simulation results. The study provides a theoretical basis for enhancing Fe removal from silicon through MACL produces, indicating that MACL methods have a potential application in deep removal of impurity Fe from silicon in the future.

Promoting electricity generation of shewanella putrefaciens in a microbial fuel cell by modification of porous poly(3-aminophenylboronic acid) film on carbon anode

We report here that the electricity generation of Shewanella putrefaciens in a microbial fuel cell (MFC) can be greatly boosted by modification of a porous poly(3-aminophenylboronic acid) (p-PAPBA) film on carbon cloth (CC) anode. The p-PAPBA/CC electrode was prepared by electropolymerization of 3-aminophenylboronic acid on SiO2 nanoparticles modified CC electrode, followed by HF leaching. The boronic acid-saccharide interactions between p-PAPBA film and bacterial cell surfaces enabled rapid biofilm growth and increased biofilm density, and the porous structure of p-PAPBA film facilitated the mediated extracellular electron transfer of Shewanella putrefaciens via self-secreted flavins, all of which resulted in promoted electricity generation of Shewanella putrefaciens in the MFC. A maximal power density of 928 ± 20 mW m−2 was generated by the MFC using the p-PAPBA/CC anode, which was 4.4-fold that of the MFC using the CC anode. This study provides a promising way to promote the electricity generation of Shewanella putrefaciens in MFCs by making full use of the boronic acid-saccharide interactions.

Chemical purification processes of the natural crystalline flake graphite for Li-ion Battery anodes

We investigated the effect of the acid species, the liquid-to-solid ratio, the reaction time, and temperature on the carbon content (CC) of natural crystalline flake graphite in Ethiopia. Specifically, after physical purification through a froth flotation, CC was observed to be 94.71 %. However, when the acid-alkali-acid, H2SO4/H2O2-NaOH−HCl, chemical purification process was employed in this study, CC was significantly improved to 99.68 % (or 99.72 % after additional HF leaching), based on both a loss-on-ignition test and thermogravimetric analysis. Here, the purified graphite powder exhibited a conductivity of 21.9 S/cm, placing it in the range of semiconductor materials. The hexagonal structure of graphite, when analyzed by X-ray diffraction, was independent of the purification process. However, according to the scanning electron microscopy image, the impurities in the graphite sample were removed effectively through the purification process. Finally, based on the overall material properties of this purified graphite, this natural flake graphite could be used for high tech applications such as Li-ion battery anodes.

Effect of Acid Leaching on Silicon Nanoparticles and Their Electrochemical Performance in Lithium-Ion Batteries.

In this work, crystalline Si nanoparticles are synthesized on a large scale via a low temperature molten salt method. The crystal morphologies and electrochemical properties of the samples after HCl and HF leaching are studied in detail. The electrochemical properties of the as-produced silicon samples, which are used as anode materials in lithium-ion batteries (LIBs), are evaluated. The final product, which contains more Si and less SiO², improves cyclic stability because it buffers the volumetric effect of Si during lithiation/delithiation. The HF acid leach removes the impurity of SiO². However, the prepared sample exhibits lower electrochemical properties. The sample with a low SiO² content can deliver a capacity of 1503 mAh g-1 after 50 cycles at a higher current density of 1 A g-1, and the Coulombic efficiency is approximately 100%. However, the sample after HF acid leaching only delivers a discharge capacity of 389 mAh g-1 at 1 A g-1 after 50 cycles.

Effect of Sodium Borate on the Preparation of TiN from Titanomagnetite Concentrates by Carbothermic Reduction–Magnetic Separation and Acid Leaching Process

Carbothermic reduction–magnetic separation and acid leaching processes were used to produce TiN and direct reduced iron (DRI) from titanomagnetite concentrates. The effects of sodium borate on the reduction behavior of TMCs, the magnetic separation of the reduced products, and the purification of the impure TiN by acid leaching were investigated. Results of x-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy analysis showed that magnesium aluminate spinel (MgAl2O4) was generated in the reduced products, which could hinder the purification of the TiN. Adding sodium borate not only inhibited the formation of MgAl2O4, but also promoted the formation of TiN by decreasing the roasting temperature and time. Adding sodium borate slightly affected the separation of metallic Fe and TiN. By adding 16% sodium borate, a DRI with 94.3% Fe, 0.6% Ti, and 0.1% V was obtained by magnetic separation. After HCl + HF leaching, the TiN product containing 74.1% Ti and 2.8% V was obtained with the Ti recovery of 94.6% and V recovery of 58.3%.

Simple and High-Effective Purification of Metallurgical-Grade Silicon Through Cu-Catalyzed Chemical Leaching

A simple and effective method for the removal of impurities from large-sized particle metallurgical-grade silicon (MG-Si) powders based on Cu-catalyzed chemical leaching (CuCCL) has been proposed and discussed. The leaching behaviors of the main metallic impurities (Fe, Al, Ca, Ti, Ni, V and Cu) were investigated using various leaching approaches. The typical precipitates at Si grain boundaries before and after leaching were observed and analyzed by scanning electron microscopy and energy dispersive x-ray spectroscopy. The leaching results show that the order of impurity removal efficiency, from highest to lowest, is CuCCL > HF-H2O2 leaching > HF leaching. After CuCCL, the total metal impurity concentration can be reduced from 6759 ppmw to 193.41 ppmw. The numerous micro-scale “channels” introduced by CuCCL are beneficial for the removal of impurities, especially for the non-dissolving metal impurities, such as calcium and aluminum. The results indicated that CuCCL is promising as an industrial purification method to produce solar-grade silicon.

Removal of impurities from metallurgical grade silicon with metal assisted chemical leaching

In this study, a novel metal assisted chemical leaching (MACL) was proposed to remove impurities from metallurgical grade silicon (MG-Si). HF acid leaching, one step MACL (1-MACL, HF + AgNO3) and two step MACL (2-MACL, HF + AgNO3 + H2O2) were employed to purify MG-Si. The typical precipitates at Si grain boundaries before and after leaching were observed and analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The leaching results show that the impurities removal efficiency is 2-MACL > 1-MACL > HF acid leaching. After 2-MACL, the impurities (Fe, Al, Ca, Ti, Mn, Ni, Cu, V, Cr, Ag, B and P) concentration can be reduced from 4550.40 ppmw to 106.09 ppmw and the purity of MG-Si increased from 99.55 to 99.99%. It is notable that the introduction of micro-scale “channels” can facilitate the impurities removal when compared to solely HF leaching, especially for some non-dissolving metallic impurities like copper, calcium and aluminum. It should be noted that 2-MACL has obvious effect on the removal of non-metallic impurities B and P.

Effect of acetic acid on the leaching behavior of impurities in metallurgical grade silicon

ABSTRACTTo explore the potential of acid leaching in purification metallurgical grade silicon (MG-Si) for solar cells, the effect of acetic acid on the leaching behavior was investigated in the present work by focusing on the behavior of impurities affected by the addition of acetic acid to a conventional acid mixture composed of hydrochloric acid and hydrofluoric acid. The etching results reveal that the HCl–HF–CH3COOH mixture is a better lixiviant for dissolving impurity inclusions in MG-Si. The extraction yield of impurities in HCl–HF–CH3COOH leaching was found to increase by 7% compared to conventional HCl–HF leaching.

Purification of metallurgical-grade silicon using zirconium as an impurity getter

Abstract Zr was employed as an impurity getter to enhance removal of impurities from MG-Si because of its strong affinity for a wide range of impurities. Different lixiviants, HCl + HF, HF and aqua regia, were employed to investigate the leaching behavior of MG-Si at 348 K for 2.5 h with and without Zr addition. Typical precipitates at Si grain boundaries before and after acid leaching were observed and analyzed via scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). The leaching results show that the order of efficiency of the lixiviants, from highest to lowest, is HCl + HF > HF > aqua regia. HCl + HF is the optimal lixiviant for purification of MG-Si with and without Zr addition. The presence of Zr in MG-Si can significantly enhance the efficient extraction of impurities, especially P and Al, which cannot be removed efficiently using conventional hydrometallurgical technologies. The total fraction of impurities removed (Fe, Al, Ca, B, P, Zr, Ti, V, Mn, Cu and Ni) increased from 94.2 to 97.5% after HCl + HF leaching. In addition, 99.9% of the impurity getter Zr was removed, with its concentration decreasing from 50,000 ppmw to 49 ppmw after HCl + HF leaching. The residual amount of Zr in the refined Si can be further removed by directional-solidification purification because of its extremely small segregation coefficient.

Systematic investigations of graphene layers in sub-bituminous coal

Coal is the plentiful and widely and universally used fuel. However, its structural characteristic makes a perception that it is only worthwhile for generating energy via combustion. Herein we report a simple method to synthesize nanometre-sized graphene sheets with amorphous carbon addends on the edges from sub-bituminous coal. The X-ray analysis reveals the presence of crystalline carbon in the amorphous background. The average number of carbon atoms and aromatic layers was estimated as ∼21 and 8. The interlayer spacing d 002 for the aromatic lamellae of samples leached using HF and acetic acid, are found to be 0.352 and 0.376 nm, respectively. The oxidation of coal structure resulted in nanometre sized graphene having lateral size of 4.19 nm and stacking height of 2.3 nm. The Raman spectrum analysis confirmed the formation of finite sized, less defective graphene nanolayers with leaching. The crystalline carbon within the coal matrix displaced with chemical leaching, resulting nano-meter sized graphene sheets. The results also established that with HF leaching, carbon in coal becomes more stacked and ordered compared to organic acid leached coal.

Pyroxene separation by HF leaching and its impact on helium surface-exposure dating

Cosmogenic 3He surface-exposure dating has become an important tool in earth surface sciences. Pyroxene, together with olivine, is the principal mineral phase for this technique, making 3He particularly appropriate for volcanic lithologies. However, two important factors that affect the viability of the method are the extensive, thus expensive, preparation procedure and the often under-constrained purity of pyroxene separates. Here, we present an approach to preparing pyroxenes for 3He dating, adapted from the quartz-separation method for 10Be and 26Al analyses, which utilises hydrofluoric-acid leaching to improve pyroxene purity and streamline the pyroxene separation procedure. In addition to producing abundant sample in relatively little time, the results of two experiments demonstrate that acid leaching (i) yields samples of higher purity than conventional methods, reflected in slightly elevated 3He concentrations, and (ii) reduces 4He concentrations, in turn elevating the 3He/4He ratio. This new protocol for preparing pyroxenes thus has the potential to increase the precision and accuracy of cosmogenic 3He surface-exposure dating.

Effect of HF leaching on 14C dates of pottery

This paper presents the experiments with 14C dating of two potsherds, which contained carbon dispersed rather homogeneously in their clay fabric. After AAA treatment, the potsherds still appeared to be contaminated with young carbon, presumably connected with humic acids. To make removal of humic acids more effective, we treated the sherds with HF acid of different concentration. The 14C results obtained demonstrate that HF treatment indeed helps to remove humic contaminants, but it also mobilizes carbon bound to raw clay, which may make 14C dates too old. We conclude therefore, that using a simple combination of HF and AAA treatment seems insufficient in reliable 14C dating of carbon homogeneously dispersed in the volume of potsherds.

CO2 Sorption on Coals: Contribution of Minerals and Influence of Supercritical CO2 Pre-exposure

The carbon dioxide (CO2) sorption isotherms of coals from different ranks and coals treated by HCl, HCl and HF sequentially by supercritical carbon dioxide (SCCO2) alone and with H2O (SCCO2/H2O), and by N2 with H2O (N2/H2O) are compared to study the influence of minerals in coals and the influence of SCCO2. The sorption isotherms and the maximum adsorbed amounts from Langmuir plots for different samples reveal that the contribution from the sorption of minerals is relatively small but that the minerals can act as an obstacle, preventing CO2 from adsorbing on the carbonaceous matrix. Generally, HCl-soluble minerals account for about 5–24% of the mineral content of test coals, while HCl-insoluble/HF-soluble minerals account for about 74–94%. Although the content of HCl-insoluble/HF-soluble minerals is much higher than that of HCl-soluble minerals, HCl leaching causes more change than HF leaching. It may be related to the distribution of the minerals in coal. The small variation of the Langmuir constant between parent and treated samples indicates only a small contribution from the minerals or the acid-leaching process. Similar sorption behaviors are observed for the SCCO2/H2O-treated and SCCO2-treated coal samples, as well as the untreated and N2/H2O-treated coal samples for QS and DT coals. A higher sorption capacity of the former pair indicates that the pre-exposure of coal in SCCO2 alone or with H2O leads to a similar increase in CO2 sorption capacity. H2O in pretreating does not make a significant difference in CO2 sorption capacity. No clear change on the magnitude of the Langmuir constant can be observed among the untreated and treated coal samples. It seems that the pre-exposure of coal to SCCO2 or SCCO2 with H2O induces a change on the sorption capacity but no change on the sorption equilibrium constant. The pretreating may cause a change on the sorption sites but not on the basic properties of the carbonaceous moiety.

Combination of Microwave and Ultrasonic Irradiations as a Pretreatment Method to Produce Ultraclean Coal

Coal samples from concentrate of Shahroud coal preparation plant with an ash content of 13.8% and a total sulfur content of 1.89%, without pretreatment and also with pretreatment by microwave irradiation, ultrasonic waves, and microwave followed by ultrasonic (microwave- ultrasonic), was leached with HF and then HNO3 to produce ultraclean coal (UCC) in a batch reactor. Irradiation pretreatments, in the order of microwave-ultrasonic > ultrasonic > microwave had positive effects on acid leaching with HF, especially for coarse size fractions and early in the leaching process. After HF leaching, the HNO3 complementary leaching was more effective on the microwave-ultrasonic pretreated sample, where, after processing, 95.5% ash content of the sample was removed. A comparison between sulfur content of samples before (1.89%) and after leaching (HF followed by HNO3) shows that the sulfur content of nonpretreated sample decreased to 1.26%, whereas this reduction for microwave-ultrasonic pretreated sample was more noticeable by 0.8%. The results indicated that microwave-ultrasonic irradiation can be considered as the most effective pretreatment method in the chemical leaching of coal to produce UCC.

One-pot hydrothermal synthesis of mesoporous silica nanoparticles using formaldehyde as growth suppressant

An effective one-pot hydrothermal synthesis method based on suppressed growth strategy was invented to produce mesoporous silica nanoparticles (MSNs). In a TEOS–CTAB–NH 4OH–water–formaldehyde system, formaldehyde was used as suppressant which formed a polymer shell on the outer surface of the MSNs to control the particle size to about 30 nm. A nanoporous carbon framework was obtained when precursor was carbonized in N 2 followed by a HF leaching of the silica. The pore size of the carbon was about 30 nm. The mechanism for the synthesis process was described.