Two layer ceramic membranes and their uses

Novel two layer ceramic membranes for electrolysis cells comprising on the anodic side a layer of at least one oxide selected from the group consisting of Sb.sub.2 O.sub.5, Bi.sub.2 O.sub.5, MoO.sub.3, WO.sub.3 and V.sub.2 O.sub.5 and on the cathodic side a layer of at least one oxide selected from the group consisting of ZrO.sub.2, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5 and TiO.sub.2, electrodes provided with a two layer ceramic membrane applied thereto, an electrolysis cell provided with a two layer ceramic oxide membrane and an electrolysis process wherein a two layer ceramic membrane is in the electrodic gap.

STATE OF THE ART 
During electrolysis reaction such as the electrolysis of brine to form 
chlorine at the anode and caustic soda at the cathode, the anolyte and 
catholyte are usually separated by a diaphragm or a membrane. Asbestos has 
been usually used commercially as the diaphragm and single metal oxide 
systems have been attempted to be used as a membrane material but these 
have been unsuccessful for a variety of reasons such as too high an 
isoelectric point and/or insufficient chemical stability, etc. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide a two layer ceramic oxide 
membrane for electrolysis cells. 
It is a further object of the invention to provide novel electrode 
structures supporting a two layer ceramic oxide membrane and to an 
electrolysis cell equipped with said electrode structures. 
It is an additional object of the invention to provide a novel electrolysis 
process wherein a two layer ceramic oxide membrane separates the anode and 
cathode. 
These and other objects and advantages of the invention will become obvious 
from the following detailed description.

THE INVENTION 
The novel two layer ceramic oxide membranes of the invention are comprised 
of on the anodic side a layer of at least a material selected from the 
group consisting of Sb.sub.2 O.sub.5, Bi.sub.2 O.sub.5, MoO.sub.3, 
WO.sub.3, V.sub.2 O.sub.5 and mixtures thereof and on the cathodic side a 
layer of at least a material selected from the group consisting of 
ZrO.sub.2, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, TiO.sub.2 and mixtures 
thereof. The ceramic oxide membrane is preferably applied to an electrode 
structure to form a complete unit and the electrode is preferably the 
anode but may also be the cathode. 
The electrode base material may be any electrically conductive material 
which is resistant to the electrolysis conditions for a cell such as 
graphite. When it is the cathode, it may be made steel, stainless steel, 
nickel or iron but is preferably iron or steel mesh. When it is an anode, 
it is preferably made of a valve metal such as titanium, zirconium, 
tantalum, niobium, hafnium, vanadium or alloys thereof and the active 
surface is provided with an electrocatalytic coating containing a platinum 
group metal oxide such as disclosed in U.S. Pat. Nos. 3,778,307 and 
3,711,385. The preferred anode material is titanium with an 
electrocatalytic coating of a mixed crystal material of titanium dioxide 
and ruthenium oxide. 
By mixed-crystal material is generally understood that the molecular 
lattices of the oxide of the film-forming metal are intertwined with the 
molecular lattices of the other material constituting the coating. There 
are various methods of achieving such a structure, some of which will be 
described hereinafter in connection with the processes for making the 
electrode according to the invention, but this is not intended to restrict 
the scope of the invention. 
The other material of the mixture consists of one or more representatives 
of the non-film forming conductors. This other material may consist of a 
mixture of a metal and the oxide of the metal, or of a mixture of two 
metals or of a mixture of a metal and an oxide of a different metal, or 
other permutations and combinations of conductors and oxides. Preferably 
the conductors belong to the group consisting of gold, silver, platinum, 
palladium, iridium, ruthenium, osmium, rhodium, iron, nickel, chromium, 
copper, lead, manganese, and the oxides thereof, graphite, nitrides, 
carbides and sulfides. 
The coating according to the invention need not cover the entire surface of 
the electrode to be immersed in the electrolyte. As a matter of fact, the 
coating need only cover 2% of the immersed zone, and the electrode will 
still operate effectively and efficiently. 
While the membranes may be applied to the cathode or the anode, the anode 
is preferred as the ceramic oxide membrane has a higher degree of 
adherence to the valve metal than the steel cathode material. When 
deposited on the cathode, the membrane at its surface with the steel 
cathode may be contaminated and even plugged by formation of iron oxides 
if the cell is shut down and this does not occurr at the ceramic oxide 
interface with the valve metal substrate. The membrane on the cathode can 
be plugged inside with precipitated alkaline earth metal hydroxides which 
does not occur at the anode. 
Generally speaking, the layer of the membrane on the anode side has both a 
very low isoelectric point i.e. a pH.ltoreq.2.5 which may act as the 
cation carrier and has a higher chemical stability in acid solutions. The 
layer of the membrane on the cathode side has a high chemical stability in 
strongly alkaline solutions and has a high isoelectric point, i.e. a 
pH.gtoreq.5.0. 
The two layer ceramic oxide membranes may be applied to the cathode or 
anode by any convenient means such as by plasma jet or by sintering of the 
materials at a temperature below the melting point of the electrode. 
Preferably, the oxides are applied by plasma jet as powder with an 
appropriate mesh size such as 150 to 250 mesh. The application conditions 
are well known for this procedure and may be for example 4000.degree. C. 
with a gas carrier under pressure and at a distance of 20 to 30 cm. The 
thickness of the membrane should be as thin as possible and is preferably 
about 50 to 500 .mu.m. 
Before the ceramic oxide membrane is applied the electrode base, is 
preferably cleaned and then roughened by sand blasting or acid etch to 
improve the adhesion. To avoid the ceramic oxide membrane from covering 
the active electrode surface which is on the back side or side opposite 
the membrane, the active surface is preferably protected before 
application of the membrane by a thin coat of a metal such as zinc, tin or 
aluminum or other metal which is easily removed later by melting or 
dissolution in a solvent such as 3 to 5% nitric acid or 3 to 5% sodium 
hydroxide or other appropriate means. 
The novel process of the invention for the preparation of an electrode 
structure provided with a two layer ceramic oxide membrane comprises 
cleaning the electrode, applying to the active electrode surface a thin 
layer of an easily removable metal, applying the two layer oxide membrane 
to the electrode with the phase towards the anolyte being selected from 
the group consisting of Sb.sub.2 O.sub.5, Bi.sub.2 O.sub.5, MoO.sub.3, 
WO.sub.3 V.sub.2 O.sub.5 and mixtures thereof, and the phase towards the 
catholyte being selected from the group consisting of ZrO.sub.2, Nb.sub.2 
O.sub.5, Ta.sub.2 O.sub.5 TiO.sub.2 and mixtures thereof and removing the 
protective metal layer from the active electrode surface. Mixtures of 
substantially any proportion of 2 or more of the oxides in each phase or 
layer may be used. 
The novel electrolysis cell of the invention is comprised of a cell having 
at least one electrode pair of an anode and cathode and provided with 
electrolyte inlet and outlet means, means for impressing a direct current 
on the cell, means for recovering the electrolysis products and a two 
layer ceramic oxide membrane on one of the electrodes with the layer 
towards the anolyte being selected from the group consisting of Sb.sub.2 
O.sub.5, Bi.sub.2 O.sub.5, MoO.sub.3, WO.sub.3, V.sub.2 O.sub.5 and 
mixtures thereof and the layer towards the catholyte being selected from 
the group consisting of ZrO.sub.2, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, 
TiO.sub.2 and mixtures thereof. 
The novel electrolysis process of the invention comprises electrolyzing an 
electrolyte between an anode and a cathode with a direct current, the 
improvement comprises providing one of the said electrodes with a two 
layer ceramic oxide membrane with the phase towards the anolyte being 
selected from the group consisting of Sb.sub.2 O.sub.5, Bi.sub.2 O.sub.5, 
MoO.sub.3, WO.sub.3, V.sub.2 O.sub.5 and mixtures thereof and the phase 
towards the catholyte being selected from the group consisting of 
ZrO.sub.2, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, TiO.sub.2 and mixtures 
thereof inside of the electrodic gap. The process is particularly useful 
for the production of halogens by electrolysis of aqueous alkali metal 
halides of chlorides, hypochlorites, persulfates, perborates, oxidation of 
organic compounds, etc. Particularly preferred is the production of 
chlorine by the electrolysis of brine. 
Examples of preferred two layer ceramic oxide membranes are ZrO.sub.2 
-Sb.sub.2 O.sub.5, Ta.sub.2 O.sub.5 -Sb.sub.2 O.sub.5, Ta.sub.2 O.sub.5 
-Bi.sub.2 O.sub.5, Ta.sub.2 O.sub.3 -WO.sub.3, Ta.sub.2 O.sub.5 
-MoO.sub.3, Ta.sub.2 O.sub.5 -V.sub.2 O.sub.5, ZrO.sub.2 -Bi.sub.2 
O.sub.5, ZrO.sub.2 -WO.sub.3, ZrO.sub.2 -MoO.sub.3, ZrO.sub.2 -V.sub.2 
O.sub.5, TiO.sub.2 -Sb.sub.2 O.sub.5, TiO.sub.2 -WO.sub.3, TiO.sub.2 
-V.sub.2 O.sub.5, Nb.sub.2 O.sub.5 -Sb.sub.2 O.sub.5, Nb.sub.2 O.sub.5 
-WO.sub.3, Nb.sub.2 O.sub.5 -V.sub.2 O.sub.5, ZrO.sub.2 TaO.sub.5 - 
SbO.sub.5, ZrO.sub.2 Ta.sub.2 O.sub.5 - Sb.sub.2 O.sub.5, B.sub.2 O.sub.5 
etc. 
In the following examples there are described several preferred embodiments 
to illustrate the invention. However, it is to be understood that the 
invention is not intended to be limited to the specific examples. 
EXAMPLE 1 
An expanded iron cathode base which had been sand-blasted was precoated on 
its back side with a thin layer (1 to 2 mm) of zinc by plasma jet to close 
the voids and avoid deposition of oxides thereon. The opposite surface of 
the cathode which will face the anode was provided by plasma jet with a 
first layer of zirconium oxide and a second layer of antimony pentoxide 
with a thickness of less than 150.mu. for each oxide layer. The zinc layer 
was then removed by soaking the cathode in a 5% nitric acid solution at 
20.degree. C. for 1-2 minutes and washing with distilled water to remove 
traces of acid. The resulting membrane had a zirconium oxide surface on 
the cathodic side where it is stable to the alkaline conditions of the 
anolyte and an antimony pentoxide surface on the anodic side where it is 
stable to the acid chlorinated conditions of the anolyte. The isoelectric 
point for the zirconium oxide surface was at least a pH of 5 and for the 
antimony pentoxide was very low at a pH of less than 2.3. The interface 
between the two oxide layers is a mixture of the two oxides. 
The said ceramic oxide coated electrode was placed in an electrolysis cell 
to electrolyze a sodium chloride solution of 230-300 g/l of NaCl at 
90.degree. C. and a current density of 2000 A/m.sup.2. The catholyte flow 
rate was 0.3 to 0.1 liter/hour with a head of 100 mm (H.sub.2 O). After 3 
days of operation the catholyte composition was NaOH 130 g/l - NaCl 50 g/l 
and the faraday efficiency was 90%. 
EXAMPLE 2 
The procedure of Example 1 was repeated except the base material was a 
titanium anode substrate and the first layer was antimony pentoxide and 
the second layer was zirconium oxide. The back side of the titanium anode 
structure was then provided with an electrocatalytic coating of a 
RuO.sub.2 -TiO.sub.2 mixed crystal material. The anode was then placed in 
the electrolysis cell and electrolysis was effected as in Example 1. 
EXAMPLE 3 
The procedure of Example 2 was repeated several times with the zirconium 
oxide being replaced with Ta.sub.2 O.sub.5, TiO.sub.2 and Nb.sub.2 
O.sub.5. The procedure of Example 2 was also repeated while replacing the 
antimony pentoxide with Bi.sub.2 O.sub.5, MoO.sub.3, WO.sub.3 and V.sub.2 
O.sub.5. The membranes applied on the foraminous anodes all operated 
safisfactorily in the cell. 
Various modifications of the structures and process of the invention may be 
made without departing from the spirit or scope thereof and it is to be 
understood that the invention is intended to be limited only as defined in 
the appended claims.