Preparation of chromic acid using bipolar membranes

The invention relates to a process for the preparation of alkali metal dichromates and chromic acid by the electrolysis of monochromate and/or dichromate solutions in a multi-chamber cell, wherein the anode chamber is shielded from the solution of chromate, dichromate and/or chromic acid by a bipolar ion exchanger membrane.

This invention relates to a process for the preparation of alkali metal 
dichromates and chromic acid by the electrolysis of alkali metal 
monochromate or alkali metal dichromate solutions. 
According to U.S. Pat. No. 3,305,463 and CA-A-739 447 the electrolytic 
preparation of dichromates and chromic acid takes place in electrolysis 
cells in which the electrode compartments are separated by cation exchange 
membranes. 
For the production of alkali metal dichromates, alkali metal monochromate 
solutions or suspensions are introduced into the anode compartment of the 
cell and converted into an alkali metal chromate solution in which alkali 
metal ions are selectively transferred to the cathode compartment through 
the membrane. For the preparation of chromic acid, alkali metal dichromate 
or alkali metal monochromate solutions or a mixture of alkali metal 
dichromate and alkali metal monochromate solutions are introduced into the 
anode compartment and converted into solutions containing chromic acid. 
Sodium monochromate and/or sodium dichromate is generally used for these 
processes. In both processes, an alkaline solution containing alkali metal 
ions is obtained in the cathode compartment. This solution may consist, 
for example, of an aqueous sodium hydroxide solution or, as described in 
CA-A-739 447, of an aqueous solution containing sodium carbonate. 
For the production of alkali metal dichromate or chromic acid crystals, the 
solutions formed in the anode compartments the cells are concentrated by 
evaporation; crystallisation of sodium dichromate may be carried out at, 
for example, 80.degree. C. and that of chromic acid at 
60.degree.-100.degree. C. The crystallised products are separated, 
optionally washed, and dried. 
Anode materials of lead or lead alloys as described in DE-A 3 020 260 are 
suitable in principle but discharge lead ions into the anodic solution, 
which leads to contamination of the alkali metal dichromates and chromic 
acid prepared. All so-called dimensionally stable anodes such as those 
described e.g. in DE-A 3 020 260, consisting of a valve metal such as 
titanium coated with an electrocatalytically active layer of noble metal 
or a noble metal oxide have the disadvantage that their life is limited to 
less than 100 days, in particular at elevated temperatures above 
60.degree. C. and current densities from 2-5 KA/m.sup.2. There has been no 
lack of attempts to equip such anodes based on valve metal with more 
stable catalytically active coatings, for example as described in DE-OS 3 
829 119 or DE-OS 3 905 082. Thus an increase in the service life of the 
noble metal coatings has been achieved by means of suitable interlayers. 
These interlayers are, however, by no means sufficient to render the cost 
of the anodes economically negligible as part of the cost of the whole 
process. The limit to the life of such anodes is considered (see DE-OS 3 
905 082) to be due to the fact that the valve metal is passivated by the 
permeation of oxygen through the electroactive layer, and the coating of 
noble metal is split off. 
This strictly limited service life of anodes based on valve metal is 
overcome by the process of electrolysis according to the invention for the 
preparation of alkali metal chromates and chromic acid, in which the 
anodes are only subjected to the wear of a normal water electrolysis. 
According to A. Schmidt, Angewandte Elektrochemie, Verlag Chemie 1976, 
page 123, water electrolysis with an alkaline electrolyte allows nickel or 
iron to be used. 
It has now surprisingly been found that metals or alloys such as iron or 
nickel may be used as anodes if the anode compartment is separated from 
the dichromate-containing and/or chromic acid-containing solution by a 
bipolar ion exchange membrane. A further advantage is the drastic 
reduction in the total number of electrodes required. A bipolar membrane 
is basically a combination of a cation exchange membrane and an anion 
exchange membrane. Water penetrating into the boundary layer by diffusion 
is dissociated and the H.sup.+ -ions reach the outside through the cation 
side and the OH.sup.- -ions through the anion side of the membrane. Such 
bipolar membranes have been described by 
Nagasubramanian et al, AIChE Symp. Ser. 76(192)97-104 
U.S. Pat. No. 4,355,116 of Oct. 19, 1982.

The process according to the invention is illustrated in FIGS. 1 and 2. The 
cell consists, for example, of 3 chambers (chambers 1, 2 and 3, see FIG. 
2) separated by a bipolar membrane and a cation exchanger membrane. The 
arrangement of the two membranes is shown in FIG. 1. An aqueous solution 
of alkali metal dichromate, alkali metal monochromate or a mixture of 
alkali metal dichromate and alkali metal monochromate flows into chamber 
2. Na.sup.+ -ions are selectively transferred from chamber 2 into chamber 
3 through the cation exchange membrane "K". Sodium hydroxide solution is 
formed by means of the hydroxyl ions in chamber 3 and is continuously 
withdrawn from this chamber. The Na.sup.+ -ions transferred from chamber 2 
to chamber 3 are replaced by H.sup.+ -ions which are obtained from the 
dissociation of water at the boundary surface in the bipolar membranes. 
This results in progressive acidification of the chromates, leading to the 
formation of dichromate and chromic acid. In accordance with the present 
invention, the selective behaviour of the bipolar membrane blocks the 
entrance of monochromate and dichromate ions and the entry of chromic acid 
into the anode compartment. The hydroxyl ions are derived from the 
dissociation of water within the sandwich membrane and combine with the 
protons of the decomposition of water to form water. Since on a large 
technical scale all the transports of material described only take place 
by flow of current, it is necessary to introduce charge carriers into the 
anode chamber since water itself is not sufficiently conductive for the 
electric current. The charge carrier used is a dilute sodium hydroxide 
solution which is pumped through the anode compartment. The anode 
consists, as is known, of nickel plated iron and the cathode consists of 
nickel or some other metal conventionally used in water electrolysis. The 
basic substance of the bipolar membranes preferably consists of 
perfluorinated compounds such as that described under the name of 
Nafion.RTM. for the cationic part. The anionic part has the structure 
described in EP-A 0 221 751. 
The cell illustrated in FIG. 1 constitutes only one unit. Many such 
"monocells" may be combined to form a block, FIG. 2. The combination of 
many "monocells" reduces the number of electrodes. 
Thus, in the preferred embodiment of the present invention, a plurality of 
alternating bipolar membranes and cation exchange membranes are arranged 
between the anode and the cathode of an electrolysis cell, which subdivide 
the cell into a plurality of compartments which are separated from each 
other and into which water (or dilute sodium hydroxide solution) and 
sodium bichromate solution are alternately introduced and from which 
sodium hydroxide solution and chromic acid/sodium bichromate solution are 
correspondingly removed. Dilute sodium hydroxide solution is circulated in 
the compartment containing the anode. Since gases are only produced in the 
anode-containing compartment (oxygen) and in the cathode-containing 
compartment (hydrogen), the distance between the bipolar membrane and the 
cation exchange membrane in the other compartments can be kept very small. 
According to the invention a distance of 0.5 to 2 mm between the membranes 
is sufficient. 
The electrical resistance of such a cell is essentially determined by the 
potential differences necessary for the production of hydroxyl and 
hydrogen ions. The necessary potential difference at the anode and at the 
cathode is about 2 V for the electrochemical decomposition of water, 
whereas the necessary potential difference via the bipolar membrane is in 
each case only about 0.6 V for the electrolytic Dissoziation of water, 
without consideration of overvoltages of electrode materials. 
The energy consumption of an electrolysis cell according to the invention 
is therefore all the more favourable, the more membranes are arranged 
between the anode and the cathode. 
For technical reasons a number n of from 5 to 20 bipolar membranes and from 
5 to 20 cation exchange membranes is preferred according to the invention, 
so that the electrolysis cell is subdivided into (2n+1)=11 to 41 
compartments. 
FIG. 3 shows an electrolysis cell according to the invention. It consists 
of an anode end block A, a cathode exchange membrane CM, a bipolar 
membrane BM, a cathode block C and spacers D. Elements BM, CM and D have 
through-holes in their four corners for the inflowing substance streams 
consisting of water or dilute sodium hydroxide solution and sodium 
bichromate solution and the outflowing substance streams consisting of 
sodium hydroxide solution and chromic acid/sodium bichromate solution. The 
arrows illustrate the direction of flow of the substance streams. The 
spacers D, which on the one hand provide the spacing between the membranes 
and on the other hand serve to disperse the substance streams, consist of 
a relatively coarse-meshed liquid-permeable PTFE fabric 4, onto the edges 
of which spacing and sealing surfaces 5 of a thickness of about 1 mm are 
vulcanised. Two diagonally opposing through-holes 6 of each spacer are 
sealed off from the electrolysis compartment defined by the open fabric 
surface. The other two through-holes 6 of each spacer allow diagonal flow 
through the electrolysis compartment. According to the invention the group 
consisting of elements CM, D.sub.4, BM and D.sub.3 is present from 5 to 20 
times. Sodium bichromate solution is introduced at point 1, passes through 
the adjacent through-hole of cation exchange membrane CM, and flows 
diagonally through the elecrolysis compartments defined by D.sub.4, 
whereby chromic acid is produced. The resulting chromic acid/sodium 
bichromate mixture is removed from the cell at point 8. Water or dilute 
sodium hydroxide solution is introduced at point 2 and passes diagonally 
through the (non-visible) anode compartment and the electrolysis 
compartments defined by D.sub.2. Concentrated sodium hydroxide solution is 
removed at point 9. The hydrogen gas formed at the cathode escapes at 
point 11. The oxygen formed at the cathode escapes at point 10. The device 
for pressing elements A, n(CM, D.sub.4, BM, D.sub.3) and C together is not 
shown. 3 indicates the electricity supply point for the anode. 7 indicates 
the cathode shaped in the form of a grid. 
EXAMPLE 
An experimental electrolysis cell corresponding to FIG. 3 was used in which 
the cross-sectional size of the anode, cathode and fabric 4 was 
100.times.100 mm.sup.2. The cation exchange membrane CM was Nafion.RTM. 
324 from Du Pont. The bipolar membrane used was one obtainable from WSI 
Technologies, Inc., St. Louis, Mo., USA. 
The cell contained 5 membranes of each type. A solution of 800 g/l Na.sub.2 
Cr.sub.2 O.sub.7 /l and water was introduced. The current strength was 15 
.ANG.. A voltage of between 10 and 12 V was measured between the anode and 
the cathode. The degree of acidification of the chromic acid/sodium 
bichromate solution removed was 27.4%. Also a 15% sodium hydroxide 
solution was produced which flowed out through the anode and cathode 
compartment to guarantee conductivity. 
Both the anode and the cathode consisted of iron.