Derek J. Fray

The electrowinning of lithium from chloride-carbonate melts

It is shown that lithium can be electrowon from a lithium chloride-carbonate electrolyte with current efficiencies as high as 90 pct from cells where the catholyte and anolyte are separated by a porous diaphragm and lithium carbonate is fed to the anolyte. The reduction of carbonate ions at the cathode was kept to a minimum by the porous diaphragm. The primary product of the reaction of carbonate ions with the carbon anode was carbon dioxide. Various cell designs were investigated, and a packed-bed anode consisting of a graphite tube containing a bed of graphite particles showed the greatest promise in providing a dimensionally stable current collector with preferential consumption of the bed material.

Oxidation of ceramic uranium dioxide in alkali metal carbonate-based melts: a study using various oxidants and comparison with UO2 powder

The oxidation of broken sintered ceramic UO2 pellets has been studied between 723 and 1073 K in a range of carbonate-based melts and under a variety of oxidising conditions. Comparison with the oxidation of UO2 powder under corresponding conditions showed that the reactivity of these media with ceramic UO2 was less, due to a lower relative surface area and a lower chemical activity of the sintered material. Pure carbonate melts, and with added alkali metal chlorides, produced essentially no reaction with ceramic UO2. Bubbling the melt with air or oxygen for many hours also had little effect. Addition of nitrate enabled the rate and extent of the reaction to be controlled, by varying the concentration of the oxidising agent and keeping the nitrate-to-UO2 mole ratio below ∼0.4. Above this ratio the available surface area was the main controlling factor of the reaction. Controlled in situ generation of superoxide ions increased significantly the uranate yield at 723 K. A minimum concentration of oxidising agent is thus necessary before reaction commences. Complete oxidation of ceramic UO2 was achieved in ∼2 h at 873 K in carbonate melts that contained KNO3 and were bubbled with oxygen.

Oxygen potentials in Ni + NiO and Ni + Cr2O3 + NiCr2O4 systems

The chemical potential of O for the coexistence of Ni + NiO and Ni + Cr2O3 + NiCr2O4 equilibria has been measured employing solid-state galvanic cells, (+) Pt, Cu + Cu2O // (Y2O3)ZrO2 // Ni + NiO, Pt (-) and (+) Pt, Ni + NiO // (Y2O3)ZrO2 // Ni + Cr2O3 + NiCr2O4, Pt (-) in the temperature range of 800 to 1300 K and 1100 to 1460 K, respectively. The electromotive force (emf) of both the cells was reversible, reproducible on thermal cycling, and varied linearly with temperature. For the coexistence of the two-phase mixture of Ni + NiO, δΜO2(Ni + NiO) = −470,768 + 171.77T (±20) J mol−1 (800 ≤T ≤ 1300 K) and for the coexistence of Ni + Cr2O3 + NiCr2O4, δΜO2(Ni + Cr2O3 + NiCr2O4) = −523,190 + 191.07T (±100) J mol−1 (1100≤ T≤ 1460 K) The “third-law” analysis of the present results for Ni + NiO gives the value of ‡H298o = -239.8 (±0.05) kJ mol−1, which is independent of temperature, for the formation of one mole of NiO from its elements. This is in excellent agreement with the calorimetric enthalpy of formation of NiO reported in the literature.

An electrochemical sodium sensor for aluminium melts

Abstract The modification of aluminium-silicon alloys by sodium addition before casting results in an improvement of the mechanical properties of the alloy. As sodium has a high reactivity with air, it forms sodium oxide or sodium hydroxide vapours and the total amount of sodium in the alloy decreases as a function of time. This paper describes the development of test results of an electrochemical sodium sensor for the determination of sodium in an aluminum-silicon alloy. The described experiments show the promising features of such a sensor in the aluminum casting industry, where the sodium level must be held between 50 and 200 ppm for modification of the alloy.

Phase relations in Cu-RO 1.5 -O (R = Ho, Er, Yb) and gibbs energy of formation of Cu 2 R 2 O 5 (R = Ho,Er,Yb) between 1000 and 1325K

Phase relations in Cu-RO1.5-O(R < Ho,Er,Yb) ternary systems at 1273K have been established by isothermal equilibration of samples containing different ratios of Cu:R(R < Ho,Er,Yb) in flowing air or high purity argon atmosphere for four days. The samples were then rapidly cooled to ambient temperature and the coexisting phases were identified by powder x-ray diffraction analysis. Only one ternary oxide, Cu2R2O5(R < Ho,Er,Yb) was found to be stable. The chemical potential of oxygen for the coexistence of the three phase assemblage, Cu2O + R2O3 + Cu2R2O5(R < Ho,Er,Yb) has been measured by employing the solid-state galvanic cells,< (−) Pt, Cu2O + Ho2O3+ Cu2Ho2O5//CSZ//Air (Po2< 2.12 × 104 Pa), Pt (+) (−) Pt, Cu2O + Er2O3+ Cu2Er2O//CSZ//Air (Po2< 2.12 × 104 Pa), Pt (+) (−) Pt, Cu2O + Yb2O3 + Cu2Yb2O5//CSZ//Air (Po2 < 2.12 × 104 Pa), Pt (+) in the temperature range of 1000 to 1325K. Combining the measured emf of the above cells with the chemical potential of oxygen at the reference electrode, using the Nernst relationship, gives for the reactions, 2Cu2O(s) + 2Ho2O3(s) + O2(g) → 2Cu2Ho2O5(s) (1) 2Cu2O(s) + 2Er2O3(s) + O2(g) → 2Cu2Er2O5(s) (2) and 2Cu2O(s) + 2Yb2O3(s) + O2(g) → 2Cu2Yb2O5(s) (3) δΜo2 = −219,741.3 + 145.671 T (±100) Jmol−1 (4) δΜo2 = −222,959.8 + 147.98 T(±100) Jmol−1 (5) and δΜo2 = −231,225.2 + 151.847 T(±100) Jmol−1 (6) respectively. Combining the chemical potential of oxygen for the coexistence of Cu2O + R2O3 + Cu2R2O5(R− Ho,Er,Yb) obtained in this study with the oxygen potential for Cu2O + CuO equilibrium gives for the reactions, 2 CuO(s) + Ho2O3(s) → Cu2Ho2O5(s) (7) 2 CuO(s) + Er2O3(s) → Cu2Er2O5(s) (8) and 2 CuO(s) + Yb2O3(s) → Cu2Yb2O5(s) (9) δG‡ < 22,870.3 − 23.160 T (±100) Jmol−1 (10) δG‡ < 21,261.1 − 22.002 T (±100) Jmol−1 (11) and δG‡ < 17,128.4 - 20.072 T (±100) Jmol-1 (12) It can be clearly seen that the formation of Cu2R2O5R < Ho,Er,Yb) from the component oxides is endothermic. Further, Cu2R2O5(R < Ho,Er,Yb) are an entropy stabilized phases. Based on the results obtained in this study, the oxygen potential diagram for Cu-R-O(R < Ho,Er,Yb) ternary system at 1273K has been composed.

Electrochemical sensor for measuring magnesium content in molten aluminium

Design, construction, and tests of magnesium electrochemical sensors using molten salt electrolytes and pure magnesium metal reference are described. Alumina as well as magnesia membrane thimbles were synthesized to use as the supporting matrixes for the liquid electrolytes. Binary as well as ternary salt mixtures containing M9Cl2 were impregnated in thimbles of various pore sizes. The accuracy, response time, and service life of the sensors were evaluated. The magnesium contents from the electromotive force (e.m.f.) measurements were comparable with those from the atomic absorption analysis. All sensors responded instantly to the changes of magnesium concentration in aluminium melt. The service life was between 2 to 6 h and the minimum initial equilibration time was 2 min, depending mainly on the pore size of the magnesia thimble and the electrolyte salt mixture in use.

Immobilised Molten Salt Membrane based Magnesium Sensor for Aluminium-Magnesium Melts

Magnesium sensitive probes were constructed and tested in different melts of commercial aluminium-magnesium alloys. The probes were composed of a porous magnesium oxide one closed end tube or thimble to which a magnesium conducting salt is impregnated. The activity of magnesium in the aluminium-magnesium melt was determined with report to a pure magnesium reference contained in the inside of the thimble and sealed from the surrounding atmosphere and melt by zirconia based cement. Measurements were conducted in various commercial aluminium-magnesium alloys under inert atmosphere as well as in air.

Modelling the Phenomena Associated with the Application of Centrifugal Fields in Fused Salt Electrolysis Manufacture of Light Metals

A laboratory scale fused salt electrolysis cell was constructed and successfully used to electrowin zinc.

Electrochemical insertion into and the detection of hydrogen in amorphous silicon solid electrolytes

The specific features of amorphous silicon are mainly contributed by the incorporation of hydrogen into the silicon during the vapour deposition process but, in spite of the considerable progress in the development of amorphous silicon cells, two major problems exist - light induced degradation and hydrogen evolution. As a result, there is a need to be able to measure and adjust the hydrogen content of the amorphous silicon. It is shown that this can be achieved using a potentiometric sensor, based on a proton conductor, and electrochemical tritation. Experiments show that it is possible to detect changes in hydrogen at all temperatures, using solid electrolytes, but that it is only possible to titrate hydrogen into the amorphous silicon at elevated temperatures.