Zhongning Shi

Recovery of carbon and cryolite from spent pot lining of aluminium reduction cells by chemical leaching

Abstract A two-step alkaline-acidic leaching process was conducted to separate the cryolite from spent pot lining and to purify the carbon. The influencing factors of temperature, time, and the ratio of liquid to solid in alkaline and acidic leaching were investigated. The results show that the recovery of soluble compounds of Na 3 AlF 6 and Al 2 O 3 dissolving into the solution during the NaOH leaching is 65.0%, and the purity of carbon reaches 72.7%. During the next step of HCl leaching, the recovery of soluble compounds of CaF 2 and NaAl 11 O 17 dissolving into the HCl solution is 96.2%, and the carbon purity increases to 96.4%. By mixing the acidic leaching solution and the alkaline leaching solution, the cryolite precipitates under a suitable conditions of pH value 9 at 70°C for 2 h. The cryolite precipitating rate is 95.6%, and the purity of Na 3 AlF 6 obtained is 96.4%.

Electrical conductivity of Na3AIF6-AlF3-Al2O3-CaF2-LiF(NaCl) system electrolyte

Abstract A PGSTAT 30 and a BOOSTER 20A were used to measure cell impedance. Electrical conductivity was gained by the Continuously Varying Cell Constant Technique. Electrical conductivity of KCl was measured for comparison. The results prove that the method is reliable and accurate. The electrical conductivity of Na3AIF6-AIF3-Al2O3-CaF2-LiF(NaCl) system was studied by this method. Activation energy of conductance was obtained based on the experiment results. The experiments show that electrical conductivity is increased greatly with NaCl and LiF added. Increasing 1%LiF(mass fraction) results in corresponding increase of 0.0276 S/cm for superheat condition of 15 °C. For NaCl, it is 0.024 S/cm. Electrical conductivity is increased by 0.003 S/cm with 1 °C temperature increase. The electrical conductivity is lower than that predicted by the WANG Model and higher than that predicted by the Choudhary Model.

Thermal decomposition of ammonium hexafluoroaluminate and preparation of aluminum fluoride

Abstract The thermal decomposition process of (NH 4 ) 3 AlF 6 was studied by DTA-TGA method and the related thermodynamic data were obtained. The results show that AlF 3 is obtained after three-step decomposition reaction of (NH 4 ) 3 AlF 6 , and the solid products of the first two decomposition reactions are NH 4 AlF 4 and AlF 3 (NH 4 F) 0.69 , respectively. The three reactions occur at 194.9, 222.5 and 258.4 °C, respectively. Gibbs free energy changes of pertinent materials at the reaction temperatures were calculated. Enthalpy and entropy changes of the three reactions were analyzed by DSC method. Anhydrous aluminum fluoride was prepared. The XRD analysis and mass loss calculation show that AlF 3 with high purity can be obtained by heating (NH 4 ) 3 AlF 6 at 400 °C for 3 h.

Raman spectroscopy and ionic structure of Na3AlF6-Al2O3 melts

Abstract Raman spectrum of molten cryolite was recorded. Based on the new understanding of the scattering coefficients, contents of various structural entities in acidic NaF-AlF3 melts at 942-1 024 °C in previous research were reanalyzed. The new quantitative analysis results show that when cryolite ratio(CR) is less than 2, AlF4− is the dominant anion in the melts, and its mole fraction is about 0.70 for melts with CR=1.5 and 0.50 for melts with CR=2. When CR is more than 2.5, the mole fraction of AlF63− is relatively large, which is around 0.45 for melts with CR=2.5. Ionic structure of Na3AlF6-Al2O3 melts was investigated by UV-Raman spectroscopy. Octahedral AlF63− and tetrahedral AlF4− are proved to exist with possible partial replacement of F− by O2−. Al2O2F42− with a large scattering coefficient also exists in the melts in which alumina concentration is more than 4% (mass fraction). The increase of temperature causes blue-shift of the bands in the Raman spectra.

Identification of structural entities in NdF3-LiF melts with cryoscopic method

Abstract Nd-F species in NdF3-LiF melts were studied using cryoscopic method. Liquidus temperatures of melts of various compositions were determined by differential thermal analysis(DTA). Based on the different model calculations, NdF4− was identified as the most likely Nd-F entity in the melts in which the mole fraction of NdF3 was lower than 20%, considering only one single Nd species in the melt, and which was formed in accordance with Temkin model or Flood model. Then, activities of different components in the melts were researched. The results show that activity of LiF decreases, and that of NdF3 increases with increasing the mole fraction of NdF3. The value of activity coefficient of NdF3 is higher than 1, and that of LiF is lower than 1.

Electrochemical behaviors of Mg2+ and B3+ deposition in fluoride molten salts

Abstract By using cyclic and linear sweep voltammetry, the electrochemical deposition behaviors of Mg 2+ and B 3+ in fluorides molten salts of KF-MgF 2 and KF-KBF 4 at 880 °C were investigated, respectively. The results show that the electrochemical reduction of Mg 2+ is a one-step reaction as Mg 2+ +2e − →Mg in KF-1%MgF 2 molten salt, and the electrochemical reduction of B 3+ is also a one-step reaction as B 3+ +3e − →B in KF-KBF 4 (1%, 2% KBF 4 ) molten salts. Both the cathodic reduction reactions of Mg 2+ and B 3+ are controlled by diffusion process. The diffusion coefficients of Mg 2+ in KF-MgF 2 molten salts and B 3+ in KF-KBF 4 molten salts are 6.8×10 −7 cm 2 /s and 7.85×10 −7 cm 2 /s, respectively. Moreover, the electrochemical synthesis of MgB 2 by co-deposition of Mg and B was carried out in the KF-MgF 2 -KBF 4 (molar ratio of 6:1:2) molten salt at 750 °C. The X-ray diffraction analysis indicates that MgB 2 can be deposited on graphite cathode in the KF-MgF 2 -KBF 4 molten salt at 750 °C.

Equivalent conductivity and its activation energy of NaF-AlF3 melts

Electrical conductivity of NaF-AlF3 melts was measured by continuously varying cell constant (CVCC) technique. Relationships between equivalent conductivity at 990-1030℃ and temperature and composition, and relationship between equivalent conductivity activation energy and composition of the melts were then studied on the basis of two-step decomposition mechanism of AlF6(superscript 3-). According to the changes of molar fractions of different anions in NaF-AlF3 melts, courses of dependence of equivalent conductivity and its activation energy on composition were analyzed. The results show that the influence of temperature on equivalent conductivity of the melts is small in the researched temperature range, and equivalent conductivity increases with increasing the molar fraction of AlF3; there is a minimum point in the activation energy-composition curve when molar fraction of AlF3 is 0.29.

Density, Viscosity and Electrical Conductivity of the Molten Cryolite Electrolytes (Na 3 AlF 6 –SiO 2 ) for Solar Grade Silicon (Si–SoG) Electrowinning

Density, viscosity and electrical conductivity of the molten system Na 3AlF 6–SiO 2 have been investigated in the concentration range up to 50 mole % of SiO 2. Density was measured by means of a computerized Archimedean method, viscosity of the melt by computerized torsion pendulum method and the electrical conductivity by means of the tube–cell (pyrolytic boron nitride) with stationary electrodes. It was found that the density, viscosity and conductivity vary linearly with the temperature in all investigated mixtures. On the basis of the density data, the molar volume of the melts and the partial molar volume have been calculated.

Electrical conductivity of NaF–AlF3–CaF2–Al2O3–ZrO2 molten salts

Abstract The electrical conductivity of NaF–AlF3–CaF2–Al2O3–ZrO2 system was studied by a tube-type cell with fixed cell constant. The results show that the electrical conductivity of NaF–AlF3–3%Al2O3–3%CaF2–ZrO2 molten salt system decreases with increase of ZrO2 content in an interval of 0–5%. The increase of 1% ZrO2 results in a corresponding electrical conductivity decrease of 0.02 S/cm, and the equivalent conductivity increases with the increase of molar ratio of NaF to AlF3. When the temperature increases by 1 °C, the electrical conductivity increases by 0.004 S/cm. At last, the regression equations of electrical conductivity relative to temperature and ZrO2 are obtained by quadratic regression analysis.

Mechanism of Dissolution Behavior of the Secondary Alumina

The study on dissolution of alumina in aluminum electrolyte has never been suspended. The raw materials used in aluminum smelter are made up of primary alumina and secondary alumina. The latter is the by-product of a dry scrubbing system. The differences in dissolution behavior of these two kinds of alumina are studied in this article. A see-through cell was used for observing the dissolution behavior of alumina. It was found that the dissolution rate of the secondary alumina was significantly faster than that of the primary alumina. The effect of possible impurities in the secondary alumina on the dissolution behavior, including carbon dust, hydrogen fluoride (HF), and other fluorides (Na5Al3F14, Na3AlF6, AlF3), was investigated. The carbon dust played an important role in improving the dissolution behavior. However, fluorides and chemisorbed HF have little effect on the dissolution behavior of the secondary alumina.