Browsing by Author "Shen, Leiting"

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  • Experimental Approach to Matte–Slag Reactions in the Flash Smelting Process
    (2021) Wan, Xingbang; Shen, Leiting; Jokilaakso, Ari; Eriç, Hürman; Taskinen, Pekka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Improving metal–slag separation in pyrometallurgical processes is increasingly important. Due to the harsh conditions, direct observations of the molten phases behavior in the settler of the Outotec Flash Smelting Furnace (FSF) are not possible and the ways to improve metal settling can only be studied by simulation and modeling. This study focuses on kinetics and mechanisms of the chemical reactions between matte droplets and slag, which were investigated in laboratory-scale heat-quench equipment at a typical smelting temperature of 1300°C as a function of time in both air and argon atmosphere. The reaction mechanism in the FSF settler was formulated and results in argon atmosphere also indicate that the oxidation of cuprous sulfide by ferric ions in the slag contributes strongly to the copper losses in the slag.
  • Kinetics of Scheelite Conversion in Sulfuric Acid
    (2018-02-27) Shen, Leiting; Li, Xiaobin; Zhou, Qiusheng; Peng, Zhihong; Liu, Guihua; Qi, Tiangui; Taskinen, Pekka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Complete conversion of scheelite in H2SO4 solution plays a key role in exploration of cleaner technology for producing ammonium paratungstate. In this work, the factors influencing scheelite conversion were investigated experimentally to model its kinetics. The results indicated that the conversion rate increases with increasing temperature and reducing particle size, but is almost independent of stirring speed. Moreover, although the conversion rate increases with increasing initial H2SO4 concentration (≤ 1.25 mol/L), it decreases rapidly at 1.5 mol/L H2SO4 after 10 min due to formation of a H2WO4 layer. The experimental data agree quite well with the shrinking core model under chemical reaction control in ≤ 1.25 mol/L H2SO4 solution, and the kinetic equation was established as: (Formula presented.) (t, min). This work could contribute to better understanding of scheelite conversion in H2SO4 solution and development of a new route for ammonium paratungstate production.
  • Review of rhenium extraction and recycling technologies from primary and secondary resources
    (2021-01-15) Shen, Leiting; Tesfaye, Fiseha; Li, Xiaobin; Lindberg, Daniel; Taskinen, Pekka
     A2 Katsausartikkeli tieteellisessä aikakauslehdessä
    Rhenium is a scarce and highly important metal, which is widely used in high-temperature superalloys and platinum–rhenium catalysts due to its unique physicochemical properties. The substitution of rhenium in its applications is very limited, and there is no suitable substitute without losing essential performance. Furthermore, global extractable primary rhenium resources are predicted to deplete within 130 years. In this paper, rhenium extraction and recycling technologies from primary and secondary resources are critically classified and reviewed. Rhenium is primarily produced as a by-product in molybdenum, copper, lead and uranium production from the concentrates and ores. Rhenium is extracted from roasting fume and dust, leaching residue, and aqueous solution to produce a rhenium bearing solution. Subsequently, rhenium rich solution is generated by separation with solvent extraction, ion exchange, adsorption, membrane techniques or chemical precipitation. Finally, rhenium is produced via crystallization and reduction steps. Recycling rhenium from spent alloys and catalysts is a multi-step process combining pyrometallurgical and hydrometallurgical techniques, where its separation and the subsequent steps are similar to that of extracting rhenium from primary resources. The main challenges in rhenium extraction and recycling are the enrichment of rhenium in the production and the collection and classification of spent rhenium scrap, to identify suitable processes to recover the rhenium with a high recovery. This paper contributes to better understanding the rhenium extraction and recycling processes and enhances sustainability of rhenium production.
  • Sustainable and efficient leaching of tungsten in ammoniacal ammonium carbonate solution from the sulfuric acid converted product of scheelite
    (2018-10-01) Shen, Leiting; Li, Xiaobin; Zhou, Qiusheng; Peng, Zhihong; Liu, Guihua; Qi, Tiangui; Taskinen, Pekka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    To directly obtain solution of (NH4)2WO4 instead of Na2WO4 is the key for developing a cleaner technology of ammonium paratungstate production. In this paper, ammoniacal ammonium carbonate solution was adopted for leaching tungsten from the converted product, a mixture of H2WO4 and CaSO4 obtained by treating scheelite with sulfuric acid. The research indicates that tungsten can be efficiently extracted in form of ammonium tungstate solution with WO3 leaching yield of >99.5% under moderate leaching conditions. The WO3 leaching yield is influenced by the transformation of calcium sulfate to calcium carbonate due to forming CaWO4 through the secondary reaction between CaSO4 and (NH4)2WO4, whereas an excess (NH4)2CO3 (≥0.64 mol/L) can suppress the secondary reaction by facilitating the transformation. Additionally, the consumed ammonium carbonate can be recovered by treating the leaching residue with ammonium sulfate solution at above 70 °C. This work presents a cleaner and sustainable technique for producing ammonium paratungstate, with circulating the leaching reagents and bypassing the conversion of Na2WO4 to (NH4)2WO4.
  • Technology and theory of producing APT from tungsten concentrates by sulfuric acid conversion-ammonium salt leaching
    (2019) Shen, Leiting
     School of Chemical Engineering | Doctoral dissertation (article-based)
    Tungsten is considered as a strategic and critical raw material, due to its remarkable physical and chemical properties, wide industrial applications and non-substitutability. Ammonium paratungstate is the main intermediate product extracted from tungsten ores with metallurgical processes, intrinsically linking to the tungsten industry across all supply chain stages. An efficient and cleaner technology for producing ammonium paratungstate is crucial to the sustainable development of the tungsten industry. This work presents a novel process by the sulfuric acid conversion-ammoniacal ammonium carbonate leaching route. The complete conversion of tungsten concentrates in H2SO4 solutions can be achieved by controlling the sulfuric acid concentration and adding an oxidizing agent. The formation mechanism of the H2WO4 layer on unreacted tungsten particles is explained by isopolytungsten ions diffusion-tungsten acid deposition in the diffusion layer. The more difficulty of wolframite conversion than scheelite in H2SO4 solutions is attributed to the thermodynamics, especially the accumulation of Fe2+ and/or Mn2+ in the solutions. The kinetics of scheelite conversion in moderate H2SO4 solutions agrees with the shrinking core model under chemical surface reaction control. Subsequently, an ammonium tungstate solution is directly obtained by leaching converted products in ammoniacal (NH4)2CO3 solutions at 30 °C, with WO3 leaching yield of >99%. The transformation of calcium sulfates to carbonates influences the WO3 leaching yield by secondary reactions, which can be suppressed by an excess (NH4)2CO3 in solutions through the formation of more stable vaterite and calcite. The leaching agent can be recovered to restore the leaching system. Additionally, thermodynamic modelling of the CaSO4–H2O system is carried out to better understand the thermodynamic property of CaSO4 solution and facilitate the solutions cycle and scaling prevention, as well as the upcoming CaSO4–H2SO4–H2O system. The critically revaluated solubility data were assessed with the NPL Pitzer model by MTDATA software to optimize the Pitzer parameters. The obtained model predicts very well the CaSO4–H2O system up to 300 °C, and agrees with most published solubility data.This work makes it possible to produce ammonium paratungstate cleanly and economically, featuring circulation of the leaching reagents and bypassing the conversion of Na2WO4 to (NH4)2WO4, which will hopefully be adopted in industrial practices.   
  • Thermodynamic Modeling of Calcium Sulfate Hydrates in a CaSO4-H2SO4-H2O System from 273.15 to 473.15 K up to 5 m Sulfuric Acid
    (2020-05-14) Shen, Leiting; Sippola, Hannu; Li, Xiaobin; Lindberg, Daniel; Taskinen, Pekka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    To prevent scaling and to recycle aqueous solutions in industrial processes, the thermodynamic properties of the CaSO4-H2SO4-H2O system are studied by thermodynamic modeling with the Pitzer model. The published solubility data of calcium sulfate hydrates in sulfuric acid solutions were collected and reviewed critically. Then, the CaSO4-H2SO4-H2O system was modeled using the Pitzer activity coefficient approach from critically selected experimental data to obtain optimized parameters. The model reproduces the solubility data with good accuracy up to 5 m sulfuric acid at temperatures of 283.15-368.15, 283.15-473.15, and 298.15-398.15 K for gypsum (CaSO4·2H2O), anhydrite (CaSO4), and hemihydrate (CaSO4·0.5H2O), respectively. However, at temperatures above 398.15 K and sulfuric acid concentration above 0.5 mol/kg, the solubility of anhydrite predicted by our model deviates significantly from the literature data. Our model predicts that the solubility of anhydrite would first increase but then decrease in more concentrated sulfuric acid solutions, which is in disagreement with the experimental data showing constantly increasing solubilities as a function of increasing sulfuric acid concentration. This discrepancy has been discussed. The transformations of gypsum to anhydrite and hemihydrate were predicted in sulfuric acid solutions. With increasing H2SO4 concentration, the transformation temperatures of gypsum to anhydrite and hemihydrate will decrease. Thus, gypsum is stable at low temperatures in solutions of low H2SO4 concentrations and transforms to anhydrite at high temperatures and in concentrated H2SO4 solutions, while hemihydrate is always a metastable phase. Furthermore, the predicted results were compared with previous experimental studies to verify the accuracy of the model.
  • Thermodynamic Modeling of Calcium Sulfate Hydrates in the CaSO4-H2O System from 273.15 to 473.15 K with Extension to 548.15 K
    (2019-06-13) Shen, Leiting; Sippola, Hannu; Li, Xiaobin; Lindberg, Daniel; Taskinen, Pekka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Calcium sulfate is one of the most common inorganic salts with a high scaling potential. The solubility of calcium sulfate was modeled with the Pitzer equation at a temperature range from 273.15 to 473.15 K from published solubility data, which was critically evaluated. Only two Pitzer parameters, β(1) and β(2), with simple temperature dependency are required to model the solubility with excellent extrapolating capabilities up to 548.15 K. The stable temperature range for gypsum is 273.15-315.95 K, whereas above 315.95 K the stable phase is anhydrite. Hemihydrate is in the metastable phase in the whole temperature range, and the obtained metastable invariant temperature from gypsum to hemihydrate is 374.55 K. The obtained enthalpy and entropy changes at 298.15 K for the solubility reactions are in good agreement with literature values yielding solubility products of 2.40 × 10-05, 3.22 × 10-05, and 8.75 × 10-05 for gypsum, anhydrite, and hemihydrate, respectively. The obtained Pitzer model for the CaSO4-H2O system is capable of predicting the independent activity and osmotic coefficient data with experimental accuracy. The mean absolute average error of activity coefficient data at 298.15 K is less than 2.2%. Our model predicts the osmotic coefficient on the ice curve within 1.5% maximum error.
  • Thermodynamics of Tungsten Ores Decomposition Process Options
    (2018) Shen, Leiting; Li, Xiaobin; Taskinen, Pekka
     A3 Kirjan tai muun kokoomateoksen osa
    The thermodynamics of tungsten ore decomposition in mineral acid and alkaline solutions were studied. The published thermodynamic data of tungsten minerals were collected and assessed. The Gibbs energies of CaWO4 (−1538.43 kJ/mol), FeWO4 (−1053.91 kJ/mol), MnWO4 (−1206.08 kJ/mol), H2WO4 (−1003.92 kJ/mol), and Na2WO4 (−1455.58 kJ/mol, aq) at 25 °C were adopted in the calculation using HSC software. The results show that CaWO4 is decomposed more readily in Na2CO3 solution than in NaOH, while FeWO4 and MnWO4 are more reactive in NaOH solutions. From a thermodynamic point of view, tungsten ore decomposes easily in acid solutions despite most ΔG o T ΔGTo increasing slightly with temperature. Oxidizing Fe2+ to Fe3+ in acidic solutions facilitates decomposition of FeWO4, and the reaction of CaWO4 in H2SO4 solution occurs more easily than in other mineral acids, due to the formation of sparsely soluble CaSO4. The results fit well with the experimental data and industrial experience previously reported in the literature.
  • Wolframite Conversion in Treating a Mixed Wolframite–Scheelite Concentrate by Sulfuric Acid
    (2018-02-01) Shen, Leiting; Li, Xiaobin; Zhou, Qiusheng; Peng, Zhihong; Liu, Guihua; Qi, Tiangui; Taskinen, Pekka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Complete wolframite conversion in sulfuric acid is significant for expanding the applicability of the sulfuric acid method for producing ammonium paratungstate. In this paper, the conversion of wolframite in treating a mixed wolframite–scheelite concentrate by sulfuric acid was studied systematically. The results show that the conversion of wolframite in sulfuric acid is more difficult than that of scheelite, requiring rigorous reaction conditions. A solid H2WO4 layer forms on the surfaces of the wolframite particles and becomes denser with increasing H2SO4 concentration, thus hindering the conversion. Furthermore, the difficulty in wolframite conversion can be mainly attributed to the accumulation of Fe2+ (and/or Mn2+) in the H2SO4 solution, which can be solved by reducing Fe2+ (and/or Mn2+) concentration through oxidization and/or a two-stage process. Additionally, the solid converted product of the mixed wolframite–scheelite concentrate has an excellent leachability of tungsten in an aqueous ammonium carbonate solution at ambient temperature, with approximately 99% WO3 recovery. This work presents a route for manufacturing ammonium paratungstate by treating the mixed concentrate in sulfuric acid followed by leaching in ammonium carbonate solution.