GB825445A - Process and apparatus for the electrolytic decomposition of amalgams - Google Patents

Abstract

<PICT:0825445/III/1> <PICT:0825445/III/2> <PICT:0825445/III/3> <PICT:0825445/III/4> <PICT:0825445/III/5> An amalgam decomposition cell comprises a vessel, an amalgam electrode 20, Fig. 5, e.g. a plurality of discs mounted on a rotatable shaft, in the vessel, and a depolarization electrode 18 having a porous, sintered, electrically conductive surface resistant to the hydroxide electrolyte formed in the cell, the electrode 18 having an electrode potential more positive than that of the electrode 20. The cell is connected in series with a direct-current supply source and an electrolytic mercury cell for electrolysing brine, the electrode 18 being connected to the negative of the supply source and the amalgam electrodes 20 to the positive, whereby the decomposition cell acts as a voltaic cell in the same sense as the direct current supply source. The depolarization electrode may comprise a highly porous sintered nickel strip 1, Fig. 1, formed by sintering nickel powder on both sides of a foil and is bent into sinuous loops for supporting above the liquid amalgam in the vessel, or the electrodes 18, Fig. 5, may be formed by layers of nickel powder sintered on one or both sides of carrier plates or by highly porous sheets or foils secured, e.g. by spot welding, to the sides of discs. Iron powder may be employed for sintering instead of or in addition to nickel. The amalgam electrode 20 may comprise one or more stationary roughened or sintered surfaces over which the amalgam is trickled, e.g. comprising a fabric or perforated metal sheet coated on each side with sintered metal powder. Alternatively, amalgam may flow into and out of pockets in the surface of the electrode through pores, holes or slots. Where the amalgam electrodes are rotatable discs 24, Fig. 6, they may be mounted on a common shaft 26 with disc electrodes 23 of the mercury cell 21, a pump circulating the mercury and amalgam between the cells which are in a common casing separated by a partition 25. A conventional graphite amalgam decomposer, e.g. in the form of a wash tower, may be included in the line from the decomposition cell 22 to the mercury cell 21. In the cell 22 fixed, porous, sintered electrodes 30 depend from a cell cover 31 and are interleaved with the disc electrodes, and the electrodes 30 may be electrically subdivided at terminals 34, 35 to enable the decomposition of the amalgam to be regulated by connecting or disconnecting the terminal 35. A plurality of interleaved sintered fixed and rotatable disc electrodes 48, Fig. 11, may be assembled as a unit in a casing which is suspended from the cover 44 of the decomposition cell, a plurality of units being arranged end-to-end in the cell with the shafts 50 for the disc electrodes coupled together. Holes 57, 58 in end walls of the units allow passage of amalgam and of hydrogen respectively. Filling pieces project upwardly from the bases of the casings of the units between adjacent discs 48. A relay 66 may be connected across the direct current supply source 67 so that a fall in the supply voltage allows the relay 66 to close a circuit short-circuiting the decomposition cell 65 to prevent the circulation of undecomposed amalgam to the mercury cell 64. A time-delay relay 70 may be similarly connected and is operative to break the connection between the decomposition cell 65 and the mercury cell 64 on a fall in the supply voltage and to keep the connection broken until a predetermined time after the re-establishment of the voltage, the relay 70 meanwhile closing a circuit comprising the mercury cell 64 and the supply source 67. The current in the mercury cell should exceed that in the decomposition cell by 10% to 15%, and this may be achieved by connecting an auxiliary electrode in the mercury cell, e.g. a graphite anode, of suitable surface area, directly to the supply source, a variable resistance being included in the connection if desired.