Description of GB 850,380PATENT SPECIFICATION December 12, 1956. No. 37937/56. Application made in United States of America on December 14, 1955. Complete Specification Published October 5, 1960. |
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NOTE: |
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The present
invention relates to a novel electrode for use in
electrochemical processes and to the methods of making
and using the same; and, more particularly, to a novel
lead dioxide-coated electrode which is particularly
useful in the electrolysis of chlorates in the
manufacture of perchlorates, to its method of
preparation and to its use especially in the
electrolysis of chlorates to perchlorates. It has
long been known that aqueous solutions of chlorates,
such as the alkali metal chlorates, can be electrolyzed
to form the corresponding perchlorate in solution. For
example, in the preparation of potassium perchlorate,
common practice is to electrolyze sodium chlorate to
form sodium perchlorate, treat the sodium perchlorate
with potassium chloride to precipitate potassium
perchlorate and return sodium chloride to the chlorate
cells. The literature on the electrolytic production of
perchlorates indicates a unanimous acceptance of
platinum as the only suitable anode material. In spite
of the high capital investment involved and the
significant replacement costs due to electrochemical
attack and mechanical disintegration, platinum is still
used as anode in the major perchlorate
installations. However, for reasons apparent from the above, the search for a platinum substitute, both for perchlorate manufacture and for other electrochemical operations requiring highly resistant electrodes, has proceeded for many years. None of the materials investigated in the past, however, has been adopted commercially.
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| Graphite is too strongly attacked
to be serviceable as anode material in perchlorate
production. Silicon anodes have also been suggested for the electrochemical oxidation of chlorates. Although cold-pressed and sintered silicon anodes have been found to operate without noticeable erosion, during operation the voltage rises rapidly to a very high value due to polarization, and this polarization characteristic renders silicon unusable commercially. Magnetite has been found to be useful only at very
low current densities. Although this material shows only
a slight erosion during operation, the perchlorate is
produced at a current efficiency of only 4 to
5%.
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Lead dioxide has also, been suggested for
use as anode material in the electrolytic oxidation of
chlorates and other materials. In this case, the lead dioxide has been electroplated onto iron, steel, copper or nickel metal bases. Although the lead dioxide, especially in massive form, has a very low rate of erosion itself in the chlorate-perchlorate system, the base metals on to which it is deposited are relatively rapidly eroded since the electrolyte gains access thereto through pin holes in the lead dioxide coating. According to this
invention there is provided a lead dioxide-coated
electrode for use in electrochemical processes in which
a coating of the lead dioxide is electro-deposited
directly on a tantalum body, which is rendered
substantially oxide-free before the
electrodeposition. |
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It has been found that lead dioxide can be
anodically electrodeposited on to a tantalum metal body
to provide an anode material which, although much less
expensive than platinum, possesses markedly long life
when employed in electrolytic oxidation systems,
especially those systems which rapidly attack ordinary
metals, such as the chlorate-perchlorate system.
Tantalum metal is known to polarize anodically in most
electrolytes, and in screening tests for a platinum
substitute in the electrolytic oxidation of chlorates,
tantalum was found to polarize within seconds. However,
it was found quite unexpectedly that, in the
conventional lead dioxide plating baths, this
polarization does not occur so that deposits of lead
dioxide of very high quality and of any desired
thickness can readily be built up, by anodic
electrodeposition on the tantalum metal base. The
conventional lead dioxide plating baths generally fall
within one of three principal systems: (1) alkaline aqueous solutions of lead tartrate; (2) nonalkaline aqueous solutions of lead perchlorate, and (3) acid aqueous solutions of lead nitrate. Thus it has been found that with any of these baths massive coatings of lead dioxide of high quality can be readily deposited on tantalum. |
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The polarization phenomenon referred to
above, however, does occur with tantalum in
chlorate-perchlorate oxidation cells. In this case, the
rapid polarization of the tantalum prevents it from
becoming attacked chemically due to electrolyte gaining
access to it through the lead dioxide coating. When the
lead dioxide-coated tantalum product is employed as
anode in a chlorate-perchlorate cell, the current passes
into the cell through the lead dioxide and the tantalum
base acts as an inert core. Any current leakage through
the tantalum results in the rapid formation of a
polarizing film on the surface of the tantalum thereby
preventing chemical attack thereon. The tantalum,
therefore, never erodes or contaminates the bath. When
the lead dioxide eventually becomes eroded, the
tantalum core may be reused as base for further lead
dioxide coatings. The lead dioxide-coated tantalum anode
of the present invention may also be employed as anode
in the electrolytic oxidation of chromic sulphate to
chromic acid, in the electrolytic oxidation of sulphate
to persulphate and in the electrolysis of sodium
chloride directly to sodium perchlorates.
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In preparing the
alkaline lead tartrate bath, lead tartrate itself may be
added to water along with a suitable base, or lead
tartrate may be formed in situ in the bath. This latter
embodiment is the preferred method of preparing the lead
tartrate bath, and comprises mixing in water a soluble
tartrate other than lead tartrate, such as an alkali
metal tartrate, like sodium potassium tartrate, a base
and lead oxide (PbO). The lead oxide dissolves in the alkaline tartrate solution forming lead tartrate in solution. In this embodiment it is not necessary that stoichiometric amounts of tartrate and lead oxide be employed. For example, with sodium potassium tartrate, NaKC4H4O6.4H20, between 1 and 1.5 parts, by weight preferably between 1.1 and 1.25 parts, by weight, thereof per part of lead oxide may be used. The lead tartrate bath, as stated, is to be alkaline, and hence a soluble base, preferably an alkali metal hydroxide, including ammonium hydroxide, is employed. Sodium hydroxide and potassium hydroxide are preferred, with the former being particularly advantageous. The amount of base present in the alkaline lead tartrate bath, that is to say, the alkalinity of the bath, may vary somewhat. The prior art suggests that a total of 2.7 to 3.4 moles of alkali hydroxide be present in the bath per mole of lead tartrate. However, there is an improvement in the anodic electrodeposition of lead dioxide from a lead tartrate bath which is achieved when the total amount of alkali hydroxide present provides a mole ratio thereof to lead tartrate of at least 4.5 to 1, and the amount of alkali hydroxide present may provide a mole ratio thereof to lead tartrate of as high as 8 to 1, or higher. When the lead tartrate is formed in situ in the bath, there results 2 moles of alkali hydroxide for each mole of lead tartrate formed. This alkali hydroxide, of course, becomes part of the " total " alkali hydroxide in the bath, and must be taken into consideration in determining the amount of alkali hydroxide to provide the total mole ratio referred to above. In any event, the pH of the alkaline tartrate bath should be relatively high, pH values above 12 being particularly satisfactory. The actual concentration of the main materials of the bath at the beginning of the deposition may vary widely, although excessively high concentrations may lead to precipitation and low quality deposits. For these reasons, the concentration of lead in the bath is generally not in excess of 8% (93 grams per litre). The concentration of lead may be substantially below this, and may be as low as 1 to 2% (10 to 20 grams per litre). In preferred practice, the concentration of lead in the bath ranges between 3 and 5% (between 32 and 55 grams per litre).
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The lead perchlorate bath for
electrodepositing lead dioxide is essentially an aqueous
solution of lead perchlorate having a pH ranging from
neutral to acid, preferably acid. The bath may be
prepared by adding lead perchlorate itself to water, or
the lead perchlorate may be formed in situ in the bath.
This latter embodiment is the preferred means of
preparing the lead perchlorate bath, and comprises
adding lead oxide (PbO) to an aqueous solution of
perchloric acid. The amount of perchloric acid employed
will be at least that theoretically required to combine
with the lead oxide to provide lead perchlorate, and
preferably excess perchloric acid is employed so that
the pH of the solution is acid. Referring further to the
pH, acid pH conditions are preferred, as stated,
preferably within the range of 0.8 to 5. The concentration of lead perchlorate in the bath at the beginning of deposition may vary widely, although concentrations at or near the saturation point under the conditions encountered during operation and shutdown are highly desirable. Generally the concentration of lead perchlorate may range between 50 and 500 grams per litre, preferably between 100 and 350 grams per litre.
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The lead nitrate bath is essentially an
aqueous acid solution of lead nitrate. In preparing the
bath, lead nitrate itself may be added to water, or the
lead nitrate may be formed in situ in the bath, as by
mixing lead oxide (PbO) with aqueous nitric
acid. In connection with this latter embodiment, slow addition. of the lead oxide in finely divided form to the aqueous nitric acid, with stirring, is advantageous. In general, the more acid the lead nitrate bath, the better its operation, and pH values as low as 0.8 may be employed. At acidities appreciably greater than this, acid fuming from the bath becomes excessive, especially at elevated operating temperatures. The tantalum on to which the lead dioxide is plated is not attacked at pH values as low as this. Of course, the pH of the bath may range above this figure and may go as high as 4 or more, if desired. The bath initially made up and at the time operation commences, may have a pH somewhat higher than the above figures, since, once operation has started, the pH drops to the desired level due to depletion of lead and release 80 of nitric acid. The concentration of lead nitrate in the bath, at least at the start of the deposition operation, may vary widely. In this connection, the concentration of the lead nitrate in the bath may range from as low as 50 grams per litre to as high as the maximum solubility thereof in the bath at operating temperature, which may be as high as 700 grams per litre. In general, it is desirable, in order to produce a lead dioxide deposit of optimum homogeneity, strength and surface characteristics, to maintain high concentrations of lead nitrate in the vicinity of the anode. Accordingly, concentrations above the lower end of the range, such as at least 250 grams per litre and especially at least 300 grams per litre, are preferred.
In order to facilitate handling of the
bath during shutdown periods and during continuous
replenishing procedures, when the bath may be at or near
room temperature, it is also preferred that the
concentration of the lead nitrate not be substantially
in excess of its solubility at these lower temperatures,
which may be in the neighbourhood of 400 grams per
litre. For optimum commercial operation, the
concentration of the lead nitrate will not exceed 350
grams per litre. |
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As far as the operating temperature of
the various bath systems is concerned, in general, any
of the baths may be operated at temperatures ranging
from room temperature up to the boiling point of the
bath. The exact optimum operating temperature will, of course, depend upon the particular bath system selected. In general, however, it has been found that with any of the baths, moderately elevated temperatures, such as between 60 and 75 C., are preferred. In the preparation of the electrode material in accordance with the present invention, the tantalum metal which is to serve as the anode material base, is immersed, as anode, in the desired bath, and is connected to a suitable source of current. The shape of the tantalum base is immaterial in accordance with the broader aspects of the invention, and the tantalum may be in the form of a sheet, hollow cylinder, rod, block or the like. In copending Application (Serial No. 850379) are disclosed and claimed the deposition of lead dioxide on metal bodies presenting a high surface area relative to volume, such as metal wire, or screen. |
| The tantalum metal base, before
immersion in the lead dioxide coating bath, is
substantially freed from oxide film, which hinders the
anodic plating of the lead dioxide thereon. This oxide
film can readily be removed by abrasion, such as with
fine emery cloth or controlled grit blasting for heavier
pieces. A cathode is also provided, and the cathode material may be selected from a wide variety of conducting materials, including lead itself, and carbon. Preferably the cathode is a material non-reactive with the bath during shutdown, such as carbon. Upon completion of the circuit, lead dioxide begins to deposit at the tantalum anode, and the flow of curent may be continued until the desired deposit has been built up. As stated, although tantalum polarizes anodically in most electrolytes, it will pass current anodically in the lead dioxide plating baths discussed above. Preferably, deposition is continued until the lead dioxide is built up in a relatively massive layer, that is, in a layer of at least 1/16" thick and upwards of 1 inch thick or more. By appropriate shielding or preliminary coating with stop-off paint, the deposition of lead dioxide at selected portions of the tantalum anode may be prevented or controlled as desired. The resulting product comprises the tantalum metal base body having an electrodeposited lead dioxide layer thereover.
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Operation of any of the
various baths removes lead therefrom. Before the
concentration of lead in the bath falls to the point
where the bath no longer operates efficiently, it may be
necessary, for further deposition, to replenish the
bath. The bath may be replenished periodically or
continuously. In replenishing any of the baths more lead
oxide (PbO) may be added.
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EXAMPLE I An acid lead nitrate bath is prepared
by dissolving lead oxide (PbO) in sufficient aqueous
nitric acid to provide a concentration of lead nitrate
of 350 grams per litre and a pH of 1 to 2.2. One and
one-half grams of copper nitrate trihydrate and 1.5
grams of alkyl phenoxy polyoxyethylene ethanol (" Igepal
" CO-880) are added to the solution. (" Igepal " is a
registered Trade 9C Mark.) A substantially oxide-free
tantalum rod 1" in diameter and 9" long is immersed in
the bath to the depth of 7", and is attached as anode
to a suitable source of current. A carbon rod is also
immersed in the bath and is attached as cathode to the
same source of current.
Anode current
density 30 amperes/sq decimetre |
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EXAMPLE II
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EXAMPLE III A lead tartrate bath is prepared by dissolving in one litre of distilled water, at 40 to 60'C., 50 grams of sodium potassium tartrate (NaKC4H4O6.4H20), 25 grams of sodium hydroxide (NaOH) and 48 grams of lead oxide (PbO). The bath has a pH of 13.5. A substantially oxide free tantalum sheet 1" x 5" x 0.0125" thick is immersed in the bath and is connected as anode to a suitable source of current. A carbon rod is also immersed in the bath and connected as cathode to the same current source. The bath is heated to 70C. and the circuit is completed with an anode current density of 0.7 amperes per square foot. A glossy, dark grey deposit of lead dioxide forms on the tantalum sheet. After 21 hours of plating the weight of lead dioxide deposited is x grams. |