Mechanistic investigation into the electrolytic formation of iron from iron(III) oxide in molten sodium hydroxide

Iron(III) oxide tablets were electrolytically reduced to iron in molten sodium hydroxide at 530 °C and recovered to produce iron with 2 wt.% oxygen suitable for re-melting. The cell was operated at 1.7 V and an inert nickel anode was used. The thermodynamics and mechanism of the process was also investigated. By controlling the activity of sodium oxide in the melt, the cell could be operated below the decomposition voltage of the electrolyte with the net sequence of events being the ionization of oxygen, its subsequent transport to the anode and discharge leaving behind iron at the cathode. A reduction time of 1 h was achieved for a 1 g oxide tablet (close to the theoretical reduction time predicted by Faraday’s laws) at a current density of 520 mA cm⁻² with iron phase yields of ∼90 wt.%. The energy consumption was 2.8 kWh kg⁻¹.     

阴极电导率和孔隙率对熔盐电解钛精矿脱氧过程的影响

阴极的性能通常是和电解过程与电解效率有着密切的关系。本文中测量了焙烧温度对钛精矿阴极的电导率的影响。结果表明焙烧温度对钛精矿阴极导电性影响很大。钛精矿阴极电阻率随着焙烧温度升高和接触面积的增加而增大。另外本文也研究了阴极孔隙率对熔盐电解制备Ti-Fe合金还原过程的影响。结果表明阴极的孔隙率对还原过程有着直接的影响。孔隙率的增加有利于形成中间化合物CaTiO3,从而改善电流效率。     

Direct electrochemical preparation of Nb-10Hf-1Ti alloy

The preparation of 89Nb-10Hf-1Ti alloy by the electro-deoxidation of a porous precursor cathode made from a mixture of oxides containing the alloying elements has been investigated. The results confirmed the formation of the alloy, saturated with oxygen, after 4 h of the reduction. The X-ray diffraction (XRD) and microstructural analysis suggested that the reduction starts with the formation of niobium suboxides and calcium niobates. The calcium niobate slows down the reaction until some nonstoichiometric calcium niobate (Ca₃Nb₂O_x−8) starts to form. Hafnium was also observed to be reduced through the formation of calcium hafnate without any side reactions with the other alloy constituents. Alloy with the target stoichiometry containing 390 ppm oxygen was obtained after 24 h of electro-deoxidation.     

Synthesis of La2MgNi9 hydrogen storage alloy in molten salt

La₂MgNi₉ alloy is synthesized directly from the sintered mixture of La₂O₃ + NiO + MgO in the molten CaCl₂ electrolyte by the electro-deoxidation method at 740 °C and the electrochemical hydrogen storage characteristics of the synthesized alloy are observed. Sintering (at 1200 °C for 2 h) converts the hygroscopic La₂O₃ (by the reaction with NiO) into the non-hygroscopic La₂NiO₄ and La₃Ni₂O₇ phases. The X-ray diffraction peaks indicate that the electro-deoxidation causes LaOCl, Ni, LaNi₅ and even target phase La₂MgNi₉ to form within 2 h process time. The molten salt synthesis process ends up with the final alloy structure of 79% La₂MgNi₉ and 21% retained LaNi₅. The porous alloy structure (with approximately 31.66 m²g⁻¹ specific surface area) is beneficial for higher hydrogen storage capacity and it is observed that La₂MgNi₉ alloy has promising discharge capacity which is approximately 280 mAhg⁻¹. This work clearly indicates that the electro-deoxidation is a very effective method in the synthesis of the hydrogen storage materials.     

Electrochemical synthesis of CeNi4Cu alloy from the mixed oxides and in situ heat treatment in a eutectic LiCl-KCl melt

CeNi₄Cu alloy powders were prepared by electro-deoxidation of oxide precursors in molten KCl-LiCl at 650 °C. The reduction pathway from the mixture of CeO₂ and (Ni_0.8Cu_0.2)O to CeNi₄Cu was studied by examination of partially and fully reduced samples using XRD and SEM with EDX analyses, which were obtained by interrupting the reduction process after different times. The first stage of the reaction involved the rapid formation of Ni-Cu alloy and Ni, thereafter CeO₂ was electrochemically reduced and alloyed with Ni-Cu alloy or Ni. Interconnected nodular CeNi₄Ce particles with smooth surfaces about 2 μm were obtained after the heat treatment in situ in the molten KCl-LiCl.     

A direct electrochemical route from oxides to TiMn2 hydrogen storage alloy☆

This study is for investigating the direct electro-deoxidation of mixed TiO2–MnO2 powder to prepare TiMn2 al oy in molten calcium chloride. The influences of process parameters, such as sintering temperature, cell voltage, and electrolysis time, on the electrolysis process were examined to investigate the mechanism of al oy formation. The composition and morphology of the products were analyzed by XRD and SEM, respectively. The electrochemical property of TiMn2 al oy was investigated by cyclic voltammetry measurements. The results show that pure TiMn2 can be prepared by direct electrochemical reduction of mixed TiO2/MnO2 pellets at a voltage of 3.1 V in molten calcium chloride of 900 °C for 7 h. The electro-deoxidation proceeds from the reduction of manganese oxides to Mn, which is reduced by TiO2 or CaTiO3 to form TiMn2 al oy. The cyclic voltammetry measurements using pow-der microelectrode show that the prepared TiMn2 al oy has good electrochemical hydrogen storage property. ? 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. Al rights reserved.     

A shrinking core model for the electro-deoxidation of metal oxides in molten halide salts

A shrinking core model is developed and applied to the electro-deoxidation of metal oxides (such as TiO₂, SiO₂, NiO, Cr₂O₃ and Nb₂O₃). Among these the reduction of TiO₂ is the most complex due to reduction by formation of a number of lower oxides and perovskite formation under certain experimental conditions. Hence, TiO₂ is chosen as the model material for this reduction. First, a single stage model is adopted for the reduction of TiO₂ to Ti and it is shown that an additional term for the oxygen concentration in the shell must be added to get the accurate values of oxygen concentration unless the concentration at the exterior of the grain is zero. Subsequently, a multi-stage model for the successive reduction of titanium oxides to titanium is proposed. It uses a shrinking core of the oxides in the order starting from TiO₂ to Ti₃O₅ to Ti₂O₃ to TiO to Ti. An analytical solution is developed for the transient differential equation resulting in a series solution for the concentration of oxygen in the lower oxides. Subsequently, a solution based on the pseudo-steady assumption is also developed. It is shown that for the parameters chosen, at certain values of dimensionless applied current density, I_d, (∼0.1) the transient and pseudo-steady state solutions agree in terms of the dimensionless time it takes for the core to shrink completely. The proposed model could be applied to other metal oxides such as SiO₂, NiO, Cr₂O₃, Nb₂O₃ and other metal oxides that are reduced using the Fray-Farthing-Chen (FFC) process mechanism. This can be used for a reactor scale model or for performing a parametric study of the current density and the grain size.     

Production of NiTi shape memory alloys via electro-deoxidation utilizing an inert anode

NiTi shape memory alloys (SMA) with equiatomic composition of Ni and Ti were prepared by electro-deoxidation, in molten calcium chloride, at 950 °C. Constant voltage electro-deoxidation was conducted using a NiTiO₃ cathode, and either a carbon anode or a novel CaRuO₃/CaTiO₃ composite inert anode. Both anode materials successfully allowed NiTi shape memory alloy to be obtained. The primary difference is that molecular oxygen was produced on the inert anode, instead of environmentally undesired CO₂ greenhouse gases on the carbon anode. Indeed, it was found that carbon could successfully be substituted with conductive calcium titanate-calcium ruthenate composites for electro-deoxidation. Furthermore, DSC was used to analyze the phase transformation of NiTi shape memory alloys, with results revealing the existence of reversible martensite-austenite phase transformations during the cooling and heating process.     

Synthesis of niobium aluminides by electro-deoxidation of oxides

Niobium aluminides were produced by the direct reduction of mixtures of niobium and aluminium oxides. Pellets of the mixed oxides were made the cathode in molten CaCl₂ or a CaCl₂–XCl melt, where X was either Na or Li. The oxygen in the pellets ionised, dissolved in the melt, and was discharged at the anode. The cathode reduced to a mixture of Nb₃Al and Nb₂Al which corresponded to the ratio of Nb to Al in the original oxide pellets.