A high temperature process for recycling spent nickel-metal hydride rechargeable batteries has been recently developed at SINTEF/NTNU. The spent battery modules were first frozen with liquid nitrogen for the de-activation and brittle fracture treatment. The broken steel scraps and plastics were then separated by the mechanical classification and magnetic separation. The remaining positive and negative electrodes, together with the polymer separator, were heated to 600-800oC in order to remove the organic components and further separate the Ni-based negative electrode. XRF analyses indicate that the heat-treated materials consist mainly of nickel, rare earth and cobalt oxides. The valuable rare earth oxides were further recovered by the high-temperature slagging treatment. The waste metallurgical slags, consist mainly of SiO2 and CaO, were used as the rare earth oxide absorbent. After the high temperature slagging treatment, over 98% of nickel and cobalt oxides were reduced to the metal phase; meanwhile almost all rare earth oxides remain in the molten slags. Furthermore, EPMA and XRF analyses of the slag samples indicate that the rare earth oxides selectively precipitate in the forms of solid xSiO2•yCaO•zRe2O3. The matrix of slag phase is Re2O3 deficient, typically being less than 5 wt%. This provides a sound basis to further develop the high-temperature process of concentrating the Re2O3 oxides in slags
The wetting behavior of refractory materials by molten silicon is important in the refining and casting of silicon with respect to production of low-cost solar cells. Here we studied the wetting properties of several graphite materials by molten silicon. These materials are used in the photovoltaic industry. The sessile drop method is employed to measure the contact angles. Initially, molten silicon does not wet graphite materials. The initial contact angles measured are approximately 120 deg. Molten silicon will react with C to form b-SiC and to infiltrate the refractory. Because losses of Si should be minimized, infiltration of Si into the refractory also is a problem. Surface roughness increases the contact area between Si and refractory and thus the loss of Si. Equilibrium wetting angles of 0 deg to 31 deg are measured. With increasing surface roughness, the equilibrium wetting angles decrease. The results show that the infiltration depth of molten silicon increases with the average pore size of graphite materials.
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