Reaction equations and cell chemistry

Some Relevant species
Dichlorine Oxide Cl2O
Chlorine Dioxide ClO2
Chlorine Trioxide ClO3
Dichlorine Heptoxide Cl2O7
Hydrochloric acid HCl
Hypochlorous acid HClO
Hypochlorous ion ClO-
Chlorous acid HClO2
Chloric acid HClO3
Perchloric acid HClO4
Sodium salts
Sodium Chloride NaCl
Sodium Hypochlorite NaClO
Sodium Chlorite NaClO2
Sodium Chlorate NaClO3
Sodium Perchlorate NaClO4
There is not much to be said here that has not been said elsewhere. The reactions that occur inside a Chlorate cell are not easy explained by the non-professional to the non-professional. What actually happens, how fast it happens and what way it happens depends on a lot of factors including: pH of bulk solution, temperature, concentration of starting, intermediate and end products, stirring of cell contents, current density on electrodes and current concentration in the cell, additives in cell, ratio of total volume of cell to area of electrodes, spacing and arrangement of electrodes, type of electrodes, cell voltage...and more.
To complicate matters more the effects of each of the parameters cannot be studied in isolation as all the parameters interact with one another in a complicated way. You also have the added complication of how YOU would like your system to behave. The industrial manufacturer will be very concerned with power consumption. The Amateur will not be very concerned.
All theses factors create a lot of healthy debate.

It should be noted that the Non-Professional cell will probably be operated at a high pH (ie. no pH control). When this is the case, practically all of the Chlorate is made by electricity and therefor there will be no chemical conversion of intermediates into Chlorate in the bulk of the solution, and therefor the temperature of the bulk solution will be somewhat irrelevant. Also the maximum possible current efficiency in this situation is 66.7%
If pH control is to be realised, you will have to set up a system to drop HCl into the cell continuously. Throwing a 'slash' of 10 or 20% HCl into the cell every half day or so is not sufficient.

The diagrams below attempts to show the overall reaction in a simplified manner
See What is a mole if you don't know what a mole of a substance is.



[DIAGRAM OF REACTION EQUATIONS]

There is a good discussion of Chlorate cell chemistry in the 'Further Reading Section' , Ullman. Chlorine Oxygen acids and Salts.
See also
Comprehensive Treatise of Electro chemistry Vol. 2 Electrochemical Processing. J. O. M. Bockris, B. E. Conway, E. Yeager, R. E. White. Plenum Press, N.Y. 1981, also The Kirk-Othmer Encyclopedia of Chemical Tech.

 [SIMPLE CHLORATE CELL CHEMISTRY]

 
[DIAGRAM OF RELATIVE CONCENTRATION OF SPECIES IN A CHLORATE CELL]

Diagram on left shows the dependency of the concentration of intermediate species in the chlorate cell on pH. At a low pH the cell will generate Chlorine gas which will escape out of the cell and this will cause the pH to increase. As the pH rises the ease with which Chlorine can escape out of the cell gets less and less and the cell will stabilize at a pH of about 9 or 10. At this high pH, practically all Chlorate is made by electricity by the Anodic Oxidation of the Hypochlorous ion (Anodic Chlorate formation) at a maximum possible current efficiency of 66.66%, the nine electron route. Adding acid lowers the pH which will decrease the concentration of the Hypochlorous ion and increase the concentration of the Hypochlorous acid. Thus it will be possible for Chlorate to form in the bulk of the solution by these species combining (Chemical Chlorate formation), the six electron route.

Controlling the pH

The pH of the cell increases if you do not add acid to the cell. This is caused by Chlorine gas escaping from the cell. As the pH rises up to about 9 or 10 the Chlorine is no longer able to escape and the pH will stabilize.
If you want you can control the pH of your cell. This will dramatically increase current efficiency and also give much much less erosion if you are using Graphite anodes. It will decrease erosion on all Anode typed IMO. There is little point in throwing in a 'slash' of acid (say) twice a day in a effort to control pH. You will have to set up a system to drop dilute HCl into the cell. I have heard that 1% solutions are used in industry, but you could probably use stronger. pH stats are used by industry. A pH probe that is permanently immersed in the cell and that is capable of withstanding cell condition will have a high cost.< The best strategy for the amateur is to simpy drop/pump HCl into the cell at a rate that will hold pH in the wanted region. The pH is checked periodicaly using an standard pH meter.
The cell chemistry changes dramatically when the pH is held at the optimum and the optimum cell design changes too. When you are controlling pH it is good to have a fairly large bulk of solution in your cell so that there is plenty of space away from the anode for the bulk reactions to take place. Industrial set up often have two parts to a Chlorate cell. The first part has the electrodes closely arranged so that the solution spends minimal time between the electrodes. The solution (now rich in intermediates) flows into a bigger tank where bulk reactions take place.
When the cell is pH controlled the current efficiency goes up as temperature goes up. As a rough figure you can expect a 6% increase in current efficiency as you increase the temperature of your cell from 30 to 80C.

If you are not controlling pH there is little point in having conditions favorable to bulk reactions IMO. Most of the Chlorate will be made by electricity alone, therefor temperature will not have much effect on current efficiency. Note that Magnetite anodes will not make Chlorate at sensible CE if there is no pH control.

 [DIAGRAM SHOWING CURRENT EFFICIENCY VERSUS pH OF THE BULK SOLUTION.]  [DIAGRAM SHOWING HCl USAGE VERSUS TEMPERATURE OF CELL]


This table shows the effect of pH control on current efficiency and acid consumption
Quality of
pH control
pH drift Current efficiency, %35% HCl consumed
kg/ton - NaClO3
Medium 6.0 to 7.874.487.5
Good 6.7 to 6.884.562.5
From: Electrochemical Technology. 6 (1968) 402. Magnetite anode was used.


 [DIAGRAM SHOWING CURRENT LOSS DUE TO CHLORINE ESCAPING FROM CELL VERSUS pH OF CELL]

As can be seen from the above diagrams and tables, it is very beneficial to operate a pH controlled cell at a high temperature (assuming it will not erode the anode). Not alone does this increase current efficiency directly (by helping the bulk reaction take place) but it also lowers the amount of acid required to produce a given amount of Chlorate.
It is also beneficial to control the pH accurately. This increases current efficiency directly and it also decreases the total acid requirement for a given amount of Chlorate produced.
The acid requirement for a given amount of Chlorate produced will go down as current efficiency goes up.
With the pH of the cell at the optimum, the quantity of Chlorine escaping from the cell will be greater than a similar cell run at a pH of ~9-10 (pH not controlled). This represents a current loss (ie. some current is wasted producing Chlorine for no good reason). It also represents a molar loss. The extra amount of Chlorine escaping from the cell is very small as can be seen from the graph.



Figures from industry vary greatly as to the amount of acid used per KG Na Chlorate produced with a range from 40 to 110 Kilo of 35% HCl per Ton of Sodium Chlorate produced (40 to 110 grams 35% HCl per KG Na Chlorate).
If you assume that a CE of (say) 85% will be achieved in your cell you can calculate the amount of acid to add, per amp per hour, that equates to the figures in KG acid per Ton Na Chlorate. The actual figures calculated from the data above give an addition rate of 12% HCl of from 0.062ml to 0.17ml per amp per hour. (Assuming CE of 85%, density of 12% HCl 1.058g/cc).
Some actual practical figures used in amateur cells are:
  • 0.35ml per hour per amp, 12% HCl (Magnetite Anode in a very small Na cell running one amp).
  • 0.13ml per hour per amp, 12% HCl (Graphite Anode in a 2L Na cell running five amps).
  • 0.09ml per hour per amp, 12% HCl (Lead Dioxide Anode with no additive in a 2.3 liter cell running 6 amps).
  • Quote from Swede:
    HCl: With "T-Cell Jr" (KCl, 19L, 50 amps) I had the dosing timer set up to turn on 6 times per day for one minute, and each cycle of the dosing pump delivered 12-15 ml, so somewhere around 100 ml per day of 21% acid worked. I could probably have added more, the pH was more often than not around 7.5 rather than 6.8, but I was very pleased at the stability. Once it was "forced" down to near neutral, there was no tendency to rapidly climb; periodic acid dosing as a concept works, and I believe it is a good alternative to full pH control with an immersed probe, with its associated probe poisoning problems.
    [This works out at approx. 0.146ml 12% HCl per amp per hour. An MMO Anode was used.]

It would appear that small cells require more acid than larger cells. Also approx. three times more acid is required to keep pH under contol at the start of the cell run, decreasing to the above 'steady state' amounts after approx. two days. See the Graphite and LD Anode sections for some examples of pH controlled cells. When Chloride level gets to approx. 50 to 40 grams per liter the pH will stay around the wanted band without any addition of acid. This may be a useful indication of the end of the cell run.
Note that if a large excess of acid is added to the cell it will produce large amounts of Chlorine gas which may be a danger. Bear this in mind and set up pumping so that only a certain maximum of acid can enter cell should the timer malfunction and/or use good ventilation.
Various schemes have been suggested for adding acid to cells. A pump and timer as above is good or a small reservoir and a solenoid on a timer would suffice. There are some simple gravity systems here.

Cell additives

Various additives are used in the Sodium Chlorate and Perchlorate cell.
Sodium Dichromate is the main additive used in industrial cells. It is not suitable for use with Lead Dioxide. It has several roles.
Fluoride in the form of Sodium Flouride plays a similar role to Chromate and is compatible with Lead Dioxide. Fluorine also raises the Oxygen over-potential at the Anode (less Oxygen gets produced) giving greater CE in both Chlorate and Perchlorate production. Several sources have stated that Titanium Substrate Lead Dioxide Anodes and also MMO Anodes can be damaged by Fluoride containing electrolyte, therefor Fluoride is not recommended when using Ti substrate Anodes.
Persulphate is also said to increase CE with Lead Dioxide.
Yttrium is also stated to have a similar role to Chromate and is not toxic (Thesis of Linda NylÚn).
Cells that have been run by Amateur operators have shown that additives do not contribute a large increase in CE and can be left out without any great consequence.




HIT THE BACK BUTTON ON YOUR BROWSER

BACK TO TOP