Note: Descriptions are shown in the official language in which they were submitted.
1321~7~
T 726 CAN
FLUID BED ELECTROLYSIS CELL
This invention is concerned with a fluidized bed electrolysis
cell of improved design, as well as with the use of such an elec-
trolys~s cell, especially for the dissolution of metal particulates
to prepare metal salt solutions.
Fluidized bed electrolysis cells are known in the art, cf.
US-A 4,244,795 and "Chemistry and Industry", 1st July 1978,
p 465-467. The fluidized bed electrolysis cells described in these
references comprise a particulate metal cathode, one or more
conventional anodes and one or more diaphragms, preferably the
latter are conceived as tubes or pipes surrounding the anodes. The
particulate cathode is fluidized by ad~usting the flow of
catholyte, a convenient method for assessing the state of
fluidization is by measuring bed expansion. One or more current
feeders, e.g. wires, rods, strips, plates, tubes or pipes, that are `
dipped into the particulate cathode, ensure adequate distribution
of current over all metal particles. In addition to the fluidized
bed electrolysis cells described above, it is also possible to use
a particulate metal anode, together with one or more conventional
cathodes and one or more diaphragms, preferably the latter con-
ceived as tubes or pipes surrounding the cathodes. The particulate
anode is fluidized by ad~usting a flow of anolyte. One or more
current feeders, e.g. wires, rods, strips, plates~ tubes or pipes,
that are dipped into the particulate anode, ensure adequate distri-
bution of current over all metal particles.
It will be appreciated that the fluidized bed electrolysis
cell may be provided with a particulate metal cathode as well as
with a particulate metal anode.
Whilst it has been proposed to employ f1uid bed electrolysis
using particulate cathodes for the winning of metals from suitable
electrolytes such as hydrometallurgical process streams, most of
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the practical development work that has been carried
out to date has been directed towards another use,
i.e., the removal o~ metal ions from waste water
streams. As a result of the electrowinning of metals
by fluid bed electrolysis is to date at best at the
initial stage of development and no practical
commercial method is available today~ Fluid bed
electrolysis using particulate metal anodes may be
used for the preparation of metal salt solutions b~
dissolution of the particulate anode-metal.
One of the problems associated with the
electrowinning of metals is the need for an
undisturbed continuous operation. Deposition of metal
on parts or elements of the cell other than the
particulate cathode can lead to interruption of the
smooth operation of the cell and continued deposition
in undesired locations may lead to shortcircuiting of
the cell or immobilisation of the fluidized bed of
cathode particles, it also adversely affects efficient
use of current. Particularly undesirable is the
deposition of metal on the current feeders.
One of the problems associated with the
dissolution of particulate metal anodes is the need
for an insoluble current feeder to allow undisturbed
continuous operation.
The present invention is therefore concerned with
means for improving the operation of fluidized bed
electrolysis cells, particularly when these are
employed for the preparation of metal salt solutions.
Thereto this invention provides a fluidized bed
electrolysis cell comprising one or more particulate
anodes provided with one or more current feeders
carrying on their surfaces a protective film of valve
metal oxide.
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According to the invention there i8 provided
a fluidized bed electrolysis cell comprising: one or
more particulate anodes provided with one or more
current feeders carrying on their surfaces a
protective film of valve metal oxide, one or more
cathodes, one or more anode compartments and one or
more cathode compartments, each anode compartment
being associated with an anode, and each cathode
compartment being associated with a cathode, and one
or more diaphragms for separating the one or more
anode compartments from the one or more cathode
compartments.
In another aspect of the invention there is
provided a process for the dissolution of metals by
fluid bed electrolysis in an electrolysis cell of the
invention.
Valve metals are defined in this specification to
comprise any and all metals or metal alloys which may
form a protective oxide layer. Depending on the
particular application envisaged suitable cathode
valve metals are à.o. Al, Bi, Ge, Hf, Mg, Mo, Nb, Ta,
Sn, Ti, W and Zr. Preferred are Ta, Ti and Zr.
Depending on the particular application envisaged
suitable anode valve metals are a.o. Al, Mg, Nb, Ta,
Ti and Zr, particularly Ta, Ti and Zr.
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A method for constructing the special current feeders to be
applied in this invention is by employing the feeder as anode in an
electrolysis cell with an electrolyte consisting, for instance, of
a dilute oxidizing mineral acid, such as sulphuric acid. This
technique, known in the art as "anodizing", will produce - by
oxidation of the valve metal on the surface of the current feeder -
a protective film of the valve metal oxide which is coherent,
non-porous and well-adhering to the surface, such film being
referred to herein as "anodic" film. It will be appreci~ted that
the core of the current feeder may be constructed from a different
materal than the valve metal forming the surface of the current
feeder. The core may be constructed for instance, from another
~etal, or from graphite. When anodizing ~he current feeder a
suitable anode potential is 1 to 30 V, preferably 1.5 to 10 V.
The anodic films on anode feeders can also be formed in situ.
The valve metal oxide film can also be formed by suitable
chemical oxidation processes, for instance programmed temperature
oxidation in an oxygen containing atmosphere.
Investigations by the Applicants have shown that the thickness
of the oxide surface layer has a clear influence on the performance
of the current feeder used in the particulate electrode. They have
also found that the thickness is closely related to the
anode-potential applied during anodizing, the higher this
potential, the thicker the metal oxide deposit.
Examples
Testing of current feeders was carried out in a fluidized bed
electrolysis system of 8 l capacity. Electrolyte was circulated
from a central holding tank through a cell of rectangular cross-
section (~ 1.5 l capacity) that was divided into two compartments
(cathode and fluidized bed anode) with a diaphragm of 10 ~m pore
size.
The current feeders of the material tested consisted of 2 mm
diameter wires insulated with heat-shrunk pvc tubing leaving only a
surface area of 2.0 cm2 uncovered. 3 Feeders were used in the cell
in a triangular arrangement with one nearest the diaphragm.
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Titanium feeders had been anodized at 2,5 and 20 ~ anode-potential
for three minutes, while tantalum and zirconium feeders had been
anodlzed at 10 V, each for 20 minutes, all in deoxygenated
0.5 mol.l H2S04 electrolyte.
The cell was operated at a bed expansion of 27% (measured by
observing the bed height), at a nominal current density on the
beads of 1 mA.cm 2 (a current of 5.0 A). The cell was run for 6
hours. Then the feeders and the granules were withdrawn, washed
with water and acetone, and air dried before weighing to determine
the amounts of metal dissolved from the feeder and from the
granules.
The particulate anode contained Cu-beads, and a Ti current
feeder was used. The cathode was a Cu-plate and a polyethylene
diaphragm was used. The electrolyte was of nominal concentration of
100 g/l H2S04 and 10 g/l Cu. The Ti feederplate was prepared as
described above or in situ anodized in the fluidized bed
electrolysis cell. After addition of the Cu-beads the anodic
dissolution was carried out with quantitative current efficiency.
~o dissolution of the current feeder occurred,
Application of the novel electrolysis cell of this invention
for the preparation of metal salt solution involves the dissolution
of particulate metal anodes. This may be effected baechwise or in
continuous operation, in the latter event metal particles e.g.
beads, shot or chopped wire, are more or less continuously
introduced into the anode compartment. Gas evolving from the
cathode compartment is also continuously withdrawn from the cell.
The cell would normally be operated at room temperature,
although elevated temperatures, e.g. up to 70 C, may also be
employed, especially in case that the solubility of the metal salt
to be prepared is relatively low. The electrolyte solution is
circulated through the anode chamber at flow rates that would give
a bed expansion of 0 to 50~, usually up to 20~.
All kinds of particulate anode metals may be used, for
instance Cu, Zn and Sn, provided that the metals will dissolve
under the conditions employed. The metal salt solution obtained may
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be used for electrodepositing purposes (electrorefining) as
described above, or for other purposes.
Anolyte concentration may vary widely. Metal concentrations
may be obtained for instance in the case of the preparation of
Cu-solutions of up to 40 g/l. A typical anolyte will comprise from
35 to 135 g H2S04, preferably 50 to 100 g.
Cell voltage and electrode potentials are adjusted to the
various electrolytes and electrodes employed, those skilled in the
art will appreciate which combinations can be employed. Selecting
the right values forms no part of this invention since the prior
art on electrolysis contains enough guiding information.
Since the invention solves the problem of undesired
dissolution of metal current feeders, the life time of the cell is
dramatically increased, and continuous operation for several months
is possible.
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