CA1039227A - Anode coated with magnetite and the manufacture thereof - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An anode coated with magnetite is manufactured by electrodepositing iron on a metallic substrate consisting of titanium, zirconium, tantalum, or niobium and the like in an electrolyte containing ferrous sulfate, dipping said iron coated substrate into a solution of ammonium ferric oxalate under a reduced pressure, and then heating said treated substrate in an atmosphere of a gaseous mixture of hydrogen and steam.
The anode manufactured in this way is quite suitable for the pro-duction of chlorine, chlorates, and bromates, and, furthermore, it is also usable for electrolytic oxidation processes in general and as an anode for electro-winning of copper, for electrolysis of sodium sulphate, for cathodic protection, and for electro-dialysis.

Description

~039~:~7 A magnetite anode has hitherto been manufactured by casting as a hollo~ cylinder or a hollow plate, but dis-advantages of the product were inferior workability, limited shapes available and inferior electric conductivity. An improved anode coated with magnetite was proposed in order to eliminate these disadvantages by allowing iron containing iron -oxide to deposit ~ectrolytically on a substrate consisting of ~ ~
... .
`r iron or titanium, or by coating a solution containing iron . 10 .
compound which generates iron oxide on heating, followed by sub~
jecting the treated substrate to heat treatment in a gaseous !~, atmosphere consisting of a mixture of hydrogen and steam.
Although the disadvantages limited in shapes and inerior electric conductivity of the product were eliminated by this improvement, . ., ~ .
other disadvantages due to coarse-grained and less durable ~ ~
$ . : .' magnetite coating which leads to an extremely short life of the ~ product remained. Such a product, therefore, can not be accepted 3 as a satisfactory product for industrial use.
i . ': ~ ' SUMMARY OF THE INVENTION

:; . . ; , An essential object of this invention is to provide a process for manufacturing an anode coated with magnetite with industrially advantageous performance.
,. . .
Another object of this invention is to provide a process ~i for forming a fine-grained and durable magnetite coating on a ;~ metallic substrate.

Still another object of this invention is to provide ` a process for manufàcturing an anode coated with magnetite having an industrially advantageoùs life.

The aforementioned objects, other objects and the merits .;~. :, :: : . . , . . : . .
.: ,.: ,: , - ~: -. . .

~3~Z27 1 of this invention will be ~ade clear by the description here-under.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
.., . . _ . _ This invention relates to a process for manufacturing . .
an anode coated with magne-ti-te which comprises electrodepositing iron on a metallic substrate of an electrically conductive, `~ corrosion-resistant metal such as titani~, zirconium, tantalum, or niobium by using an electrolyte containing ferrous sulfate, then dipping said iron deposited substrate in a solution con- -~
' 10 taining about 10 _ 30 g/Q of ammonium ferric oxalate under a reduced pressure of 10 ~30 mm Hg abs., further heating said treated substrate at a temperature of between 550 and 7nOC in an atmosphere of a gaseous mixture of hydrogen and steam where-in the hydrogen conte~t is 10 ~25% by volume and the steam content is 75 ~90% by ~olume. Any commercially available materials in the orms of a plate, wire, screen or rod of titanium, zirconium, ~ ~ -tantalum, or niobium may be used as an electrically conductive, corrosion-resistant metallic substrate. -:~ :
As an electrolyte for electrodepositing iron, a sulfate -bath is preferable. When a chloride bath is used, although it , , . .
is commonly used now in industry, close control of the temper-ature, pH, and the composition of the bath is necessary in order to obtain a fine-grained and firmly deposited metal bonded to the metallic substrate. ~hen a sulfate bath in accordance with this invention is used, on the other hand, satisfactory results of the elèctrodeposition can always be obtained under a wider range of the conditions of electrodeposition. In addition to this, ~

the sùlfate bath is iess readily oxidized by the air than the ~ -chloride bath, and therefore said electrodepbsition can be carried Oue in the ~bsence of the ferric salt. $he presence of a '~
.~:
.~

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1~392'~7 1 ferric salt accelerates, the generation of hydrogen on the electrodepositing surface, causing undesirable formation of pin-holes, lowering the cathodic current efficiency and resulting in -~less adherent deposits, as is described in Japanese Patent : ~:
No. 219829. The sulfate bath in accordance with this invention ~
contains no ferric salt and gives an essentially high cathoaic `
current efficiency, thus assuring a satisfactory deposit with few pinholes an`d a good adherence.
The composition of the electrolyte for electrodepositing ~i iron consists of, for instance, 100 ~ 150 g/Q of ferrous sulfate ~heptahydrate~, 100 g/Q o~ ammonium sulphate and about i~
3 ~ 10 g/Q of additives such as an ammonium salt of an oryanic acid, phenol, formalin, or hexamethylene tetramine, etc.
Electrodeposition is carried out at a temperature o~ 10 ~ 40C
and at a cathodIc current density of 1.0 ~ 2.5 A/dm for about 7 ~ 20 min. Under these conditions the atmospheric oxidation `~
of the ferrous salt in said electrolyte is retarded, and almost no hydrogen is generated at the electrodepositing surface, !',`"'",' -'.: ' ~ . ' .
thus, a uniform and fine-grained deposit is obtained.
;s . ~ , As a subsequent treatment following said electro-r deposition, the iron deposited substrate is dipped into a solution of ammonium ferrlc oxalate ~ ~NH4¦3Fe~C2O4)3 ~.
Although no such treatment to an electrodeposited surface with iron has hitherto been known, it is considered as a kind of ~ - . . ..
sealing treatment which reinforces electrodeposits. I~ has ~-been confirmed that by said treatment the durability of the final product as an anode is remarkably increased as compared to one to which said treatment is not applied. Said treatment is performed by dipping the iron deposited substrate in a sealing solution containing about 10 ~ 30 g/~ of ammonium ferric i, :
.~ ~
! ; :
... .

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~3g~27 1 oxalate, at about 10 ~ 25C, then by allowing said iron deposited substrate to stand in said solution for about 20 min under a ~-reduced pressure of 10 ~ 30 mm Hg abs. A concen-tration of ammonium ferric oxalate below 10 g/Q is too dilute to be effective, and above 30 g/Q, an undesirable deposition of crystals will occur. Further, if the pressure is higher than . 30 mm Hg abs., the replacement of the air within pinholes with said solution is only imperfectly accomplished and only unsatis-: factory results are obtained. The minimum pressure is :Limited ...
~O by the saturated vapor pressure of the sealing solution. ~mmonium ferric oxalate permeated iron deposit is decomposed by a heat treatment as hereunder described, and generates gaseous ammonia and carbon dioxide, leaving only iron oxides. There are no particular difficulties in the subsequent operations.
A desired composition of magnetite and the best . ..
electric conductivity and the corrosion-resistant property of .: . :
the product can be obtained by carrying out said heat treat.-ment while maintaining conditions strLctly within the range ; ~
prescribed in the foregoing. ~ -.. 20 Said heat treatment is preferably carried out at a temperature ranging from 550 and 700C. The reaction is very ; slow at a temperature below 550C, requiring longer time for the . .
operation. At a tempèrature higher than 700C, more ferrous oxide forms and the desirable magnetite composition may not be .~. obtained. The preferable range of composition of the ~.
gaseous mixture in which said heat treatment is to be carried ~; out is from 10 to 25~ by volume of hydrogen and from 75 to 90% ~:
by volume of steam. In an atmosphere of a gaseous mixture , , .
containing less than 10~ by volume of hydrogen and more than 90%

30 by volume of steam, more ferric oxide will form, on the other ;~
, ~
-4 - ~ :

~(~392Z7 -`
1 hand, when the hydrogen content is higher than 25% by volume and the steam content lower than 75% by volume in said gaseous ~; mixture, more ferrous oxide will form. In either of these cases, the desirable magnetite composition may not be or is difficult to be o~tained. Moreover, when the hydrogen content is higher than 25% by volume, difficulties due to the hydrogen ;
embrittlement of the metallic substrate may arise.
The gaseous hydrogen-steam mixture of this invention is prepared by saturating hydrogen with steam by pas~ing hydrogen in water maintained at an appropriate temperature decided by ;~
taking the vapor pressure. Said gaseous mixture is introduced into a tube furnace to perform said heat treatment~ The time required for said heat treatment is about 2 ~ 5 hr. Finally, ` ;~
a magnetite-coated layer of 3 ~ 20 ~ in thickness is obtained.
By carrying out the treatment in accordance with the ~3 process of this invention a fine-grained magnetite-coated `
i~ layer having desirable durability may be formed on the surface i of a metallic substrate having high electric conductivity and sufficient corrosion-resistance. In particular, said sealing treatment of the iron deposit in an ammonium ferric oxalate solution pxoduces a final product as anode having a life of one year, more than twice as that whereb~ no such treatment has been applied, thus offering a practical advantage.
The anode coated with ma~netite manufactured in accordance with this invention is quite suitable for the manu-facture of chlorine, chlorates, and bromates. Moreover, said anode can be used for any electrolytic oxidation processes in general, and as an anode for elQctro~winning of copper, as an insoluble anode for electrolyzing sodium sulphate, :Eor cathodic protection, and for electro-dialysis.

.. . .

~03~31ZZ7 1 The embodiment of this invention will be ~xplained further in detail by the Examples and the Comparative Examples ; ;
- described hereunder.
Example 1 .,, ~ .
A previously polished titanium plate 200 mm x 50 mm and 1 mm tbick was deatted in a boiling 10% NaOH solution, and i was dipped into a 5% hydrofluoric acid solution at room tem-perature for 1 min, and then washed with water. Electro~
deposition was carried out for 19 min in an electrolyte at 25C
consisting of 130 g/Q of ferrous sulfate ~heptahydrate), 100 g/~
of ammonium sulfate and 6 g/Q of formalin by using said titanium plate as a cathode ~a cathodic current density = 2.5 A/dm2) ` and a low carbon steel as an anode. After the electrodeposition was completed, said iron deposited titanium plate was washed ; , . , ~ ., well with water, and then dipped into a solution of 20 gjQ of ammonium ferric oxalate at 13C, and was allowed to stand for . ~
20 min under a reduced pressure of 15 mm Hg abs. produced by a vacuurn pump and dried under the reduced pressure and was then ;~
~i 20 subjected to heat treatment at 650 C for 2.5 hrs. in an atmosphere of a hydrogen/steam gaseous mlxture aonsisting of 20% by volume of hydrogen and 80~ by volume of steam prepared by passing hydrogen into hot water at 94C. On the surface of the product the for-mation of a magnetite-coated layer was clearly recognized. The thickness of said layer was confirmed to be 20 ~ by weighing said product. The appearance of said product was uniformly I black and fine-gràined and no crack was observed therein~ - -¦ Example 2 A tantalum plate of 200 x 50 mm and 2 mm in thickness ; ;~
was defatted in a boiling 10% NaOH solution and was dipped in 1 6 ~ ~

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,:

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an aqueous solution of 5~i hydrofluoric acid for 1 min. It was then washed with water, and was electrodeposited for 10 min in an electrolyte at 20C consisting of 100 g/Q of ferrQus ` sulfate (heptahydrate~, 100 g/Q of ammonium sulfate, 3 g/Q
- of phenol, and 5 g/Q of ammonium phthalate by using said tantalum plate as a cathode ~cathodic cur:rent density = 1.0 A/dm ~, -with the use of a low carbon steel as an anode~ After the ;~
electrodeposition was completed, said iron deposited tantalum plate was washed well with water, and was then dipped into a solution containing 30 g/Q of ammonîum ferric oxalate at 23C, , and was then allowed to stand for ~ min under a reduced pressure of 28 mm Hg abs. ~fter said treatment was finished, ~ it was dried in vacuum and was heat-treated at 580C for ,.~ , 4 hrs. in an atmosphere of a hydrogen~steam gaseous mixture consisting of 15% by volume of hydrogen and 85~i by volume of steam prepared by passing hydrogen into hot water maintained -~;! at 95 _ 96C. By this treatment a uniform and fLne-graine~ "
magnetite-coated layer of 4,6~ in thickness was obtained~

Example 3 A 200 x 50 mm titanium net having a ~ire diameter of ~ 1 mm and mesh of 1 mm was defatted in a boiling 10% solution `! of NaOH, and was dipped into an aqueous solution of 5% hydro-fluoric acid at room temperature for 1 min, and then washed.
ElectrodepositLon was carried out for 15 min in an electrolyte maintained at 35C consisting of 150 g/Q of ferrous sulfate (heptahydratel, 100 g/Q of ammonium sulfate and 10 g/Q of ammonium citrate by using said titanium net as a cathode (cathodic current density = 2 A/dm ] and a low carbon steel as an anode. After said electrodeposition was completed, it was washed well with water~ and was dipped into a solution containing ...

:~
, . . .

~03'92~ -1 25 g/Q of ammonium ferric oxalate at 10C, and was allowed to stand for 20 min under a reduced pressure of 12 mm Hg abs. The treated substrate was then dried in vacuum and was heated at 670C for 2 hrs. in an atmosphere of a hydrogen/steam gaseous mixture consisting of 20% by volume of hydrogen and 80% by volume of ;
steam prepared by passing hydrogen into hot water at 94C. By this treatment a magnetite-coated layer of about 13 ~ in thickness was obtained.

Example 4 An electrolyte consisting of 250 g/~ sodium chloride, -~
70 g/Q of sodium chlorate, and 2 g~Q of sodium bichromate was electrolyzed at 60C for 11 months with an anodic current ;~
density of 10 A/dm by using an anode coated with magnetite `
~ prepared as described in Example 1 as an anode with the use ;l o~ a mild steel plate as a cathode. The current e~ficiency was .. ~ , .
~1 85~ and the average cell voltage was 3.38 V. The re~uired energy ~ ~`
per ton sodium chlorate was 6,000 kwh. During this period almost no change was observed on the surface of the anode~

Comparative Example Table 1 ;~
Time of the The State of the Surface ~ Anode Electrolysis* of the Anode ,1 ' An anode prepared by Almost no change was ;~
3~ the process of this 11 months observed.
invention ~Example 1 ' An anode prepared by A stripping off of the --~ the process of this magnetite-coated layer -~, invention except that 4.5 months was distinctly observed, dipping in an aqueous and the substrate was ~, solution of ammonium laid bare.

;'~ 30 ., '~

1 ferric oxalate was ~39zz7 , An anode prepared by The stripping off of the : :
the process of this magnetite-coated layer invention except that 2 months was slightly observed, the heat treatment was but the surface was carried out at 800C. colored brown.
- . _ ~
An anode prepared by the Same as above~
.... .
process of this invention except that heat treatment 3 months -~
10 was carried out at 450C.
.~ ' '''''''' '' ~ .
.,, '- ' .
., * Same conditions of electrolysis as in Example 4.
.. . . . .
~ , , l Sea wa~er having a sodium chloride concentration o~
`i~ 27 g/Q, pH = 8, and 25C in temperaturè, was electrolyzed by ' ........................................ . i.
. an anodic current density of 3 A/dm~, using an anode coa*ed with magnetite prepared by the process described in Example 3 with the use of ai mild steel as a cathode~ A solution con- :
'i 20 taining 0.5 g/Q of sodium hypochlorite was continuously obtained~ ~:
Even after 6 months no abnormalities were observed on said :, :
~' coated anode. ~ ~

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..... . .. ... . .

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for manufacturing an anode coated with magnetite comprising electrodepositing iron onto an electric conductive corrosion-resistant metallic substrate, said substrate being at least one selected from the group consisting of titanium, zirconium, tantalum and niobium, in an electrolyte containing ferrous sulfate, dipping said iron deposited substrate into a solution containing about 10 ~ 30 g/? ammonium ferric oxalate for about 20 min. under a reduced pressure of 10 ~ 30 mmHg abs., and then heating said treated substrate at 550 ~ 700°C for about 2 ~ 5 hrs. in an atmosphere of a hydrogen/steam gaseous mixture consisting of hydrogen of a content in a range of 10 ~ 25% by volume and steam of a content in a range of 75 ~ 90% by volume.

2. A process for manufacturing an anode coated with magnetite according to claim 1 wherein said composition of the electrolyte containing ferrous sulfate consists of 100 ~ 150 g/?
of ferrous sulfate (heptahydrate), 100 g/? of ammonium sulfate, and about 3 ~ 10 g/? of an additive selected from a group consisting of ammonium salts of an organic acid, phenol, formalin and hexamethylene tetramine, and mixtures thereof.

3. A process for manufacturing an anode coated with magnetite according to claim 1 wherein said electrodeposition is carried out under a bath temperature in a range of 10 ~ 40°C, a cathodic current density of 1.0 ~ 2.5 A/dm2, and the electro-deposition time is about 7 ~ 20 min.