CA1214747A - Method for electrochemical generation of alkaline peroxide solutions - Google Patents

Abstract

ABSTRACT
A method of producing alkaline peroxide, and a solution for use therein, comprising electrolyzing an alkaline solution in the presence of oxygen, in an electrochemical cell. The solution used contains a transition metal ion complexing agent which may be ineffective in complexing transition metal ions in the strongly alkaline solutions used. Such method inhibits reduction in current efficiency of the electrochemical cell during long term operation. In addition, the method may be advantageously used to increase current efficiency in an electrochemical cell previously used for producing alkaline peroxide with a solution which did not contain a metal ion complexing agent, and which cell is suffering from substantially reduced current efficiency.
The pH of the solutions used is typically greater than or equal to about 13.

Description

METHOD FOR ELECTROCHEMICAL GENERATION OF
ALKALINE PERO~IDE SOLUTIONS

FIELD OF THE INVENTION

This inventions relates to a process for production of alkaline peroxide solution by electroreduction of oxygen in alkaline solution, wherein the rate of loss of current efficiency apparently due to reactor deterioration, can be decreased.

DESCRIPTION OF PRIOR ART

Generation of alkaline peroxide by electroreduction o~
oxygen in alkaline solution, is described in a number of works, including U.S. Patents No. 3,454,477 and 3,607,687 to Grangaard;
U.S. Patent No. 3,969,201 to Oloman et al; Berl, W.G., Trans.
Electrochem. Soc. 83,253 (1943), and 79,359 (1939); Mizumo, S. et al.~ J. Electrochem. Soc. of Japan 17, 262-288 (19~9); Schumb, W.C., "Hydrogen Peroxide", Reinhold, N.Y. 1955; and McIntyre, J.A. and Phillips, R.F.~ "Electrolytic Synthesis of Hydrogen Peroxide in a Trickle Bed Cell", Paper no. 399, 161st meeting of the Electrochemical Society, Montreal, Quebec, May 9-14, lg82.
However, none of the foregoing, other than the McIntyre et al paper, appear to disclose results of long-term experiments (several months of continuous operation). It has been found though, that when the foregoing process is carried on for periods of several weeks in the same reactor, the current efficiency for generation of peroxide gradually decreases to levels which make -Lhe process uneconomic. Even the McIn yre and Phillips papers do no. apparently disclose such a problem. Such reactor degeneration is particularly serious when the electrolyte feed consists of commercial grade reagents (e.g. sodium hydroxide) and water which is not highly purified.
The reason for decreasing current efficiency, is not f ~ r ~ p Q /
particularly clear. The ~6~e reactions involving peroxide in the foregoing process on carbon, are generally considered to be as follows:
O H 0 2 ~ OH- - (1) H02 + H20 + 2e ~ 30H (2) 2H202~ 2H2 -t 2 ~eaction 3 is the homogeneous or heterogeneous decomposition of peroxide, which is catalysed by heavy metals (e.g. lead, iron, manganese) and '.heir compounds, and is particularly favoured in alkaline solution. While eaction 3 may con~ribute to peroxide losses, it does not explain a continually decreasing current efficiency of the reactor. Such reaction would only account for a constant inefficiency factor. A more likely explanation for the continually decreasing current efficiency, is the slow _S'4~Op~^~, 55`
modification of the carbon electrode surface to either ~ s reaction (1) or enhance reaction (2).
It is desirable then to have a method which inhibits reactor degeneration as described, and which does not reyuire overly frequent undesirable reacto-- shut-down.
It has been known previously to stabilize peroxide solutions by adding a complexing agent thereto, at least with solutions up to a pH of about 11. However, no prior attempts to 7~7 stabiliæe peroxide in solutions of pH 13 or more have apparently been made. In fact it is unlikely that any useful stabilization could be obtained with complexing agents in such alkaline solutions, since it is known that complexing agents such as DTPA
are as a rule ineffective in complexing transition metal ions in solutions of such alkalinity.

SUMMARY OF THE INVENTION

The present invention provides a method, and a solution for use therein, of producing alkaline peroxide which comprises electrolyzing an alkaline solution in the presence of oxygen (which can be provided in an oxygen containing gas) in an electrochemical cell. The solution contains a metal ion complexing agent which is effective at a pH of about 10.
However, this does not mean that the complexing agent must be effe~tive at the pH of the solution (typically about 13 or above), and in fact some of the complexing agents used are ineffective in complexing transition metal ions at the pH used.
In a particular application of the method, successi~e quantities of the solution are electrolyzed in an electrochemical cell, previously used for producing alkaline peroxide by the foregoing process but with a solution which did not contain a metal ion complexing agent, and which cell is suffering from substantially reduced current efficiency.
Preferably, the complexing agent is of a type which is effective to complex chromium, nickel, or particularly iron ions~

at a pH of at least 10, although the pH of the solution is at 7~7 least about 13. The concentration of the complexing agent in the solution is preferably less than about .5% (which percentage throughout this application, is measured as a weight percentage which ~he complexing agent forms of the total weight of the solution), and more typically is less than about .2%, although preferably at least about .002%~ with the concentration of the iron ions therein being less than about 8 ppm. Particularly where concentrations of the complexing agent below about .2% are used with a solution having an iron concentration greater than about ~ ppm, it is preferred that the solution is first filtered through a filter of no greater than about 1 micron, prior to passing the solution through the cell.
The above method is typically performed in an electrochemical cell which has a carbon bed cathode, and may have at least one electrode comprising at least one transition metal, typically iron, chromium, or nickel. Such latter electrodes are typically stainless steel electrodes. Preferred complexing agents are DTPA (diethylenetriamine pentacetic acid), EDTA
(ethylene diamine tetracetic acid), TEA (triethanolamine), or phosphate. The solution may also usefully contain at least about .002% of a polyoxyethylene alcohol type wetting agent.
The method additionally can usefully comprise the step of periodically flushing the cell with an acid solution, preferably both a nitric and oxalic acid solution.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

An electrochemical reactor (the term "reactor" bein~

74~

used to include a single cell) was constructed ln accordance with Figure 1 of U.S. Patent No. 3,969,201 to Oloman et al. The reactor of Examples 1-8 below had the following specifications:
Anode and cathode feeder electrode - 316 stainless steel Cathode bed - carbon fibre mat 1.8m long x 0.04m wide x 0.003m ~hick Diaphragm - polypropylene felt This reactor was operated under the following conditions:
Reactor feed 1.6M sodium hydroxide , commercial grade in solution tap water previously de-ionised to 1 megOhm;
at 30 ml/minute;
Commercial oxygen at 1 litre/minute STP
Reactor pressure OUT 14.6 Atm Reactor temperature IN 20C, OUT 60C
Current 78 Amperes (lkA m 2) The solution also contained .002% of a polyoxyethylene alcohol type wetting agent, to wet the diaphragms so as to keep reactor electrical resistance down. The commercial grade sodium hydroxide contained 10 ppm Fe, which diluted in the feed solution to 1 ppm. The above reactor was operated under the further parameters speci~ied in the examples below.

Example 1 The reactor was initially operated with an inlet pressure of 18.0 Atm and with a voltage of 1.5 to 1.6 volts. On start-up the reactor produced 30 ml/minute alkaline solution containing 0.64 M peroxide to give a current efficiency for peroxide of 80%. Over a 4 week period of continuous operation ~LV~7~

the current efEiciency for peroxide gradually fell to 64V/o.
Treatment with 5% nitric acid restored the efficiency to 70% but this subsequently fell to 50% in the next 8 weeks of continuous operation. At this point~DTPA was added to the feed solution to give a concentration of 0.2%. In 3 days the peroxide current efficiency rose to 72% and remained at that level for 4 days while 0.2% DTPA was added continuously to the feed solution.
When DTPA addition was stopped the current efficiency declined to 55% in one week of ~t-}~ operation.

It should be noted that after about the first month of operation, a one micron polypropylene filter was provided in the feed line, /~>c /~ase(l in order to relieve an ~ea~e pressure drop observed without such filter. The filter was used in all examples, except where S,a e c i f i ~ ./
sp4~6ed otherwise.

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Example 2 In the conditions as in Example 1 but without DTPA in the feed solution, the peroxide current efficiency was sustained at about 72% for two weeks by the addition of 0.16%
triethanolamine to the feed solution.

Example 3 The reactor described in Example 1 was purged with 5%
nitric acid and a new run commenced, with the conditions of Example 1, except that the feed solution contained only 0.02%
DTPA. For 4 weeks of continuous operation the current efficiency for peroxide remained in the range of 72 to 78%.

Example 4 Following the procedure of Example 1, the reactor was operated for about 2 weeks with no DTPA or other complexing ~2~79~

agen~s. Currenc efficiency fell to about 50%. Following this, .002% DTPA was added to the feed solution over a period of about 10 days. Current efficlency rose from about 52% as measured 3 days after addition, to about 55%. Following this, .02% DTPA was added for a period of about 1 week. After about 3 days, current efficiency had risen to about 70%, and increased to about 72%
shortly before the end of this trial.

Example 5 In a longer trial, current efficiency was maintained above 70% for about 100 days by .02% DTPA in the feed water.

Example 6 Following Example 5, ferrous sulphate was then added to the feed water to provide 10 ppm of Fe therein (which resul~ed in 8 ppm in the reactor feed solution), and current efficiency nevertheless was held between about 75% to 80% during an 8 day trial, with the addition of .01% DTPA in the feed water. When .he feed filter was bypassed with 8 ppm Fe being present in the feed solution, .01% DTPA alone was ineffective to prevent a drop in current efficiency and eactor plugging (which results in a higher pressure drop). However, the further addition of .01% TEA
both prevented reactor plugging, and maintained current efficiency above 73% du-ring a 4 day -trial. The combination of .01% DTPA and .02% sodium heptonate~ did not have this beneficial effect in current efficiency.
*All sodium heptonate used was supplied by Croda Synthetic Chemicals Limited, Wolverhampton, England.

~ ~7~7 Example 7 In a subsequent trial to that of Example 6, again with the feed filter bypassed and with 10 ppm Fe added to the feed water, and with ~01a/o DTPA and .01% sodium heptonate, current efficlency fell from 77 to 55% in 4 days of operation. However, pressure drop was stable and voltage increased from 1.6 to 2.1 volts. When th s trlal was continued without iron addition, the current efficiency increased from 55 to 71% dur~ng a period of 3 days, although the voltage continued to increase to 2.3 volts.

Example 8 The procedure completed in Example 6, was carried on wi.h the addition of 10 ppm Fe in the feed water, (making 8 ppm Fe ln the eactor feed solution), .01% DTPA, and .01% TEA in the feed solution. Over the 4 days of this trial, the pressure drop was stable at 3.2 atmospheres, vol~age stable at 1.45 volts, with ~he current efficiency decreasing slightly from 76 to 73%.

Example 9 The reactor used in this Example was basically the same as that used in the above Examples, except the anode and cathode feeder electrodes were 304 stainless steel, and the cathode bed was compressed graphite particles of dimensions approximately .42mm x .30mm x 1/8". In addition, this ;eactor had an active electrode dimension of 50cm x 5cm. The reactor was operated at a temperature of 25 to 40C, a pressure of 110 to 120psig, liquid flow of 10ml/min., and commercial oxygen gas flow of 1900ml/min.

-~%~7~

a, STP The reac.or feed solution was 2 M analytical grade sodium hydroxide in tap water This reactor was operated at short periods (typically several hours) at varying current densities and with either no complexing agents added, or with di-sodium EDTA or sodium pyrophosphate present in the liquid feed solution in concentrations each of 01 M The resul.s are summarized in the table below (current efEiciency shown as percentages) Current Density No Complexing EDTA (Di- Sodium (AMP/CM-) Agent Sodium Salt) Pyrophosphate 4 77% 94% 94%

8 72% 76% 77%

67% 72%

12 - 66% 71%

It will thus be seen from Example 1 that cu~rent efficiency in a reactor of the type described, will decrease over ~ime when the reactor is used in the alkaline peroxide process, and at least where the feed solution contains iron in low concentrations (1 ppm, presen-. as a result of contaminants in the commercial grade sodium hydroxide utilized) Again as shown in Example 1, such decreasing current efficiency can actually be reversed by utilizing the complexing agent DTPA over a period of ~l%~L~7~

several days. However, if use of DTPA ~as suddenly stopped as in Example 1, the current efficiency would again decline. The foregoing is also illustrated by Example 4.
Examples 3 and 5 illustrate that continuous use of DTPA, even at a concentration of .02%, will maintain a high current efficiency over a fairly long period of time. Example 2 illustrates this also in connection with the use of the complexing agent triethanolamine to the feed solution.
The resul-s of Example 6 and 8 illustrate that even with higher Fe concentrations (about 8 ppm) in the reactor feed solution, current efficiency can still be maintained with use of a fairly low concentration (.01%) of a complexing agent such as DTPA in .he feed water. However in the absence of a filter such as the 1 micron filter, it is necessary to at least raise the overall concentration of complexlng agents present. Furthermore, Examples 6 and 7 illustrate ,hat sodium heptonate does not appear to be as effective as TEA in maintaining current efficiency, at least at higher Fe concentrations.
Example 9 illustrates .hat EDTA and sodium pyrophosphate, markedly increase current efficiency even at the pH levels therein (> 13), at least in the short run.
It should be noted that there are two observed side effects from utilizing complexing agen-ts as described above.
First, such agents may ;.end to increase corrosion of the electrodes. This was observed by an analysis of the product solution from the above examples, as well as others. Such analysis tends to indicate that an average corrosion rate of the stainless steel electrodes in the above examples, would be about ~2~79~

32 grams per year, or approximately .06 mm per year of electrode thickness, depending upon the concentration of the complexing agents. Second, it was also found that peroxide decomposition in the producc solution was higher than in a product solution where no complexing agencs were added. With no added complexing agents, the decomposition rate was about .25% H2O2 per day at 22C, but with the various added complexing agents, the rate increased therefrom to about .8% H2O2 per day at 22C.
It should also be noted that the results indicate that it will still be necessary in order to hold current efficiency above 70% and reactor voltage low, as well as caustic/peroxide ra.io below 3.5kg/kg to wash the reactor about once a month w th nitric and/or oxalic acid solutions. It was found in the present case _hat the reactor pe.formed nearly as new, with current e:Eficiency of about 75 to 80%, voltage about 1.5 volts, and pressure drop at 3 atmospheres, causticlperoxide ratio at 3.1 kg/kg and specific electrochemical energy of 3kWh/kg, with a wash utilizing the following procedure. Firstg the reactor was shut down and depressurized, and purged with water for two hours. The reactor was then purged wi-ch each of a 5% nitric acid and 2%
oxalic acid solution, each for about 4 hours. This was followed by a further water purge for 2 hours, as well as a purge with 1.6 M caus,ic feed solution for 2 hours, with the reactor being pressurized and started up within about 30 minutes following such procedure.
Ic will be observed then, thac the use of such complexing agents as described, is effec-.ive even though the complexing agencs are no~ particularly effective in complexing 7~7 heavy meLal ions ln s~rongly alkaline solutions (i.e. pH 13).
Furthe-rmore, the fact that the current efficiency oE the reactor deteriorates over time, rather than remaining steadily poor, J,~q r ~' S S
indicates that deterioration of the electrodes to elthe-r ~ sS
reaction (1) as on page 2, o- enhance reaction (2) on page 2, is the likely explanation for such decreasing current efficiency.
Furthermore, thls appears to be conflrmed by the fact that addition of a complexing agent to the feed solution used in a reactor with substantially decreased current efflciency, does not immediately raise such current efficiency, but only does so over a period of several days. Thus, it appears unlikely that use of the complexing agents increases current efficiency merely by complexing heavy metals such as iron, in the reactor feed solution. This conforms with the fact that again, it is known that complexing agents in solutions of the degree of alkalinity described (> about pH 13), are generally ineffective in complexing heavy me-tal ions. As well, the fact that the decomposition rate of peroxide in product solutions from ~he reactor in which complexing agents have been used, is higher than where such complexing agents have not been used in the feed solution might tend to suggest tha. a lower current efficlency would result from such use, although as found above the opposite is the case.
As will be apparent to .hose skilled in the art in light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention ~2~
withou-t departing Erom the spirit or scope thereof. Accordingly, the scope of the inven-tion is to be construed in accordance with the substance defined by the following claims. The caustic/
peroxide ratio mentioned above is the ratio oE total titratable alkali. expressed as NaOH to peroxide conten-t expressed as H22 in the reactor product solution.

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Claims (33)

I CLAIM:

1. A method of producing alkaline hydrogen peroxide comprising electrolyzing an alkaline solution in the presence of oxygen in an electrochemical cell, which solution contains a transition metal ion complexing agent which is effective at a pH of at least 10.

2. A method as described in claim 1 wherein successive quantities of the solution are electrolyzed in an electrochemical cell, previously used for producing alkaline peroxide by electrolyzing an alkaline solution in the presence of oxygen, which solution did not contain metal ion complexing agent, and which cell is suffering from substantially reduced current efficiency, so as to increase current efficiency of the cell.

3. A method as described in claim 1 wherein the pH of the solution is at least about 13.

4. A method as described in claim 1 wherein the complexing agent is effective to complex iron, chromium, or nickel ions at a pH of at least 10, and the pH of the solution is at least about 13.

5. A method as described in claim 1 wherein the complexing agent is effective to complex iron ions at a pH of at least 10, and the pH of the solution is at least about 13.

6. A method as described in claim 5 wherein the complexing agent is ineffective to complex iron ions at the pH of the solution.

7. A method as described in claim 2 wherein the complexing agent is effective to complex iron ions at a pH of at least 10, and the pH of the solution is at least about 13.

8. A method as described in claim 3 wherein the concentration of the complexing agent in the solution is less than about .5%.

9. A method as described in claim 4 wherein the concentration of the complexing agent in the solution is less than about .5%.

10. A method as described in claim 5 wherein the concentration of the complexing agent in the solution is less than about .5%, and the concentration of metal ions therein is less than about 12 ppm.

11. A method as described in claim 8 wherein the concentration of the complexing agent in the solution is less than about .5%, and the concentration of iron irons therein is less than about 12 ppm.

12. A method as described in claim 5 wherein the concentration of the complexing agent in the solution is between about .002% and .2% and the concentration of iron ions therein is less than about 8 ppm.

13. A method as described in claim 6 wherein the concentration of the complexing agent in the solution is between about .002% and .2% and the concentration of iron ions therein is less than about 8 ppm.

14. A method as described in claim 5 comprising filtering the solution through a filter of no greater than about 1 micron prior to passing the solution through the cell.

15. A method as described in claim 12 comprising filtering the solution through a filter of no greater than about 1 micron prior to passing the solution through the cell.

16. A method as described in claim 13 comprising filtering the solution through a filter of no greater than about 1 micron prior to passing the solution through the cell.

17. A method as described in claim 3 wherein the solution is electrolyzed in an electrochemical cell having a carbon bed cathode.

18. A method as described in claim 5 wherein the solution is electrolyzed in an electrochemical cell having a carbon bed cathode.

19. A method as described in claim 12 wherein the solution is electrolyzed in an electrochemical cell having a carbon bed cathode.

20. A method as described in claim 1, 2, or 3 wherin the complexing agent is selected from the group consisting of DTPA, EDTA, TEA, or sodium heptonate.

21. A method as described in claim 4, 12 or 13 wherein the comnplexing agent is selected from the group consisting of DTPA, EDTA, TEA, heptonate, or phosphate.

22. A method as described in claim 14, 15, or 16 wherein the complexing agent is selected from the group consisting of DTPA, EDTA, TEA, heptonate, or phosphate.

23. A method as described in claim 17, 18, or 19 wherein the complexing agent is selected from the group consisting of DTPA, TEA, or heptonate.

24. A method as described in claim 17, 18, or 19 wherein the solution additionally contains at least about .002%
of a polyoxyethylene alcohol type wetting agent.

25. A method as described in claim 17, 18, or 19 wherein the complexing agent is selected from the group consisting of DTPA, TEA, or heptonate, and the solution additionally contains at least about .002% of a polyoxyethylene alcohol type wetting agent.

26. An alkaline solution of a pH of at least about 13, for use in a method of preparing alkaline hydrogen peroxide by electroreduction of oxygen, comprising .002% to .2% of a metal ion complexing agent selected from the group consisting of DTPA, EDTA, TEA or a sodium phosphate, and wherein the con-centration of transition metal ions is no greater than 10 ppm.

27. A solution as described in claim 26 additionally comprising at least about .002% of a polyoxyethylene alcohol type wetting agent.

28. A method as described in claim 1, 4, or 12 wherein the solution is electrolyzed in an electrochemical cell having at least one electrode comprising at least one transition metal.

29. A method as described in claim 1, 4, or 12 wherein the solution is electrolyzed in an electrochemical cell having at least one electrode comprising at least one of the transition metals selected from the group consisting of iron, chromium, and nickel.

30. A method as described in claim 1, 4, or 12 wherein the solution is electrolyzed in an electrochemical cell having at least one stainless steel electrode.

31. A method as described in claim 1, 4, or 12 additionally comprising periodically flushing the cell with an acid solution.

32. A method as described in claim 1, 4, or 12 additionally comprising periodically flushing the cell with a nitric acid solution and oxalic acid solution.

33. A method as described in claim 1, 4, or 12 wherein the solution is electrolyzed in an electrochemical cell having at least one stainless steel electrode, the method additionally comprising periodically flushing the cell with a nitric acid solution and an oxalic acid solution.