Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing single metal
Patent
1996-03-07
1997-11-25
Valentine, Donald R.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing single metal
205566, 205571, 205575, 205587, 205598, 205594, 204260, 204269, 204272, 204275, 204283, 204284, 204286, 204290F, 204292, 204294, 204237, C25C 108, C25C 112, C25C 120, C25C 704
Patent
active
056908067
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the recovery of metals from dilute solutions.
It is well known that the electrodeposition of metals from dilute solutions requires the use of very high mass transfer cells or high surface area electrodes, the latter operating at low current densities on a real area basis. In order to realise high surface areas previous work has involved the use of mesh electrodes, toughened electrodes and metal and carbon foams. With the introduction of conducting carbon and graphite fibres of very small diameter, typically 5 to 15 microns, these fibers can be used to provide an electrode of a very high surface area to volume ratio. The inventors have now discovered that by using a cathode comprising such carbon or graphite fibers, through which the dilute solution of metal is passed, metal ions can be removed from solution, especially metal containing effluents, in a suitably engineered electrochemical cell. For example, copper can be removed from solution to levels below 1 ppm with high current efficiencies from solutions containing initially 20 ppm.
There is described in EP-A-0 129 845 (American Cyanamid Company) an electrode comprising a plurality of continous fibers, wherein each of said fibers has a thin, firmly adherent, metallic coating thereon. The disclosed electrochemical cell is a complex arrangement, and for effluent treatment simple operations are essential for the successful commercialisation of an electrochemical metal removal cell. It is a prerequisite of any cell for removing metals from dilute solutions that the cell design is simple and the cathode easy to replace. The individual cells are desirably modular so that additional cells can be added if the metal loading in a particular application is required to be increased.
Carbon fibers can be obtained in several forms e.g. papers, veils, yarn, tow, chopped or milled fibers, needled, non-woven mat and as felts. These fibers can, therefore, be made up into a variety of forms e.g. flat felts or cylinders. Many of these carbon fibers have relatively high electrical conductivities which can be optimised depending on the heat treatment applied during the production process. Typically a single filament can have a resistivity of 3.1.times.10.sup.-3 to 22.6.times.10.sup.-3 ohm-cm.
It is an object of the present invention to provide a simple electrochemical cell that can be used to remove metal from solutions containing up to 50 ppm.
According to the present invention there is provided an electrochemical cell for the removal of metals such as copper, lead, silver, tellurium, platinum, palladium or nickel from dilute solutions of the metal, said cell comprising a porous tubular support which is provided with a cathode comprising a porous carbon fiber material, a current feeder for the cathode, a tubular anode spaced from said cathode, a current feeder for the anode, the anode and cathode being enclosed by a non-porous outer casing, the arrangement being such that in use during electrolysis the dilute solution from which the metal is to be removed is introduced into the cell by means of an inlet and flows through the porous carbon fiber cathode and out from the cell through an outlet.
The porous support is preferably fabricated from a non-conducting substance such as porous polyethylene, an open mesh structure or an appropriate filter cloth supported on the open structure so that the flow regime required can be obtained. The support may also be a conducting material in which case it can also act as the cathode current feeder.
The combination of the support and the carbon fiber cathode in intimate contact acts to control the flow distribution of the electrolyte through the cathode. A pressure drop is inevitably created when the electrolyte passes through the porous support and cathode. In order that the pressure drop and hence the flow is regulated, the relative pore sizes of the cathode and porous support may be adjusted in different embodiments of the invention. For example, when a cathode of open structure (i.e. large pore size) is employ
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Dalrymple Ian McCrady
Sunderland John Garry
EA Technology Ltd.
Valentine Donald R.
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