US Patent No. 2,813,825
Method of producing Perchlorates
Henry C. Miller and John C. Grigger
Pennsalt Corporation
Nov. 19, 1957
The present invention related to a novel method for electrolytically producing Perchlorates and more particularly the present invention relates to a novel method for electrolytically oxidizing a Chlorate aqueous solution to the corresponding Perchlorate by which improved current efficiencies during operation of the cell are realised.
It has long been known that aqueous solutions of Chlorate such as alkali metal Chlorates can be electrolyzed to form the corresponding Perchlorates in solution. The literature on the electrolythic production of Perchlorates indicated a unanimous acceptance of platinum as the most favoured anode material. In spite of the high cost due to electrochemical attack and mechanical disintegration, platinum is still used as an anode in the major Perchlorate installations. The search for a platinum substitute for Perchlorate manufacture has proceed for many years with the result that other materials have been suggested for use ass anode including graphite, silicon, manganese dioxide, magnetite, and Lead Dioxide. In copending applications Serial Nos. 552,968 and 552,970 filed Dec. 14 1955, are disclosed and claimed particularly advantageous electrodes comprising Lead Dioxide which are particularly suitable for use in Perchlorate cell.
As is known, in the electrolysis of Chlorate to Perchlorate there is a drop in current efficiency in the cell as the concentration of Chlorate in the electrolyte decreases. The exact drop, and the concentration level at which it begins to take noticeable effect varies with the particular anode employed. For example, in electrolyzing Sodium Chlorate to Perchlorate employing a Platinum anode, the overall current efficiency is generally maintained at a high value in the neighbourhood of 80% as the Sodium Chlorate is electrolyzed from about 600 g/l down to about 100g/l. In a cell with a Lead Dioxide anode, the current efficiency begins to fall even at the initially high Sodium Chlorate concentrations and although an average current efficiency of about 68% has been obtained in the Sodium Chlorate concentration range of about 600 to100 g/l, the current efficiency usually drops to below 10% in the Sodium Chlorate concentration range below 100g/l.
It has been previously been suggested to employ additives in an attempt to increase current efficiency in the Perchlorate cell. Present day commercial cells using Platinum anodes generally employ an electrolyte containing initially about 600g/l of Chlorate, usually Sodium Chlorate, and 5g/l of a soluble dichromate, usually Sodium Dichromate. There has also been a suggestion to add a small amount of Sodium Fluoride to increase the current efficiency of a cell employing a Lead Dioxide anode.
It has been found that that there are definite limitations to the use of such additives, Using a Lead Dioxide anode, for example, and Sodium Fluoride as the additive, 2.5g/l of Sodium Fluoride increased the overall current efficiency appreciably on the first batch electrolysis; however, the current efficiency steadily dropped on three succeeding electrolysis to below the value obtained before the Sodium Fluoride addition. The presence of Sodium Dichromate at concentrations as low as 0.5g/l (as the dihydrate) sharply decreased the current efficiency of the Chlorate to Perchlorate electrolysis when operation with a Lead Dioxide anode. Other materials, including Sodium Thiocyanate, Platinum Chloride, Sodium Hexametaphosphate, Sodium Pyrophosphate, Sodium Perborate, Sulfuric Acid, and Hydrogen Peroxide have also been tried resulting either in no appreciable beneficial effect or actual reduction in cell current efficiency.
It has been found that the inclusion of a water soluble Persulphate in the Chlorate containing electrolyte significantly increased the cell current efficiency during electrolysis.
At the initiation of electrolysis it is generally desired to have a concentration of Chlorate in the electrolyte as high as feasible under the temperature conditions obtaining, since the higher the concentration the higher the efficiency. In most cases this initial concentration of Chlorate is in the neighbourhood of 500-700g/l. As the electrolysis proceeds, the concentration of Chlorate as such in the electrolyte diminishes until a point in reached, usually in the neighbourhood of 2-20g/l of Chlorate, where further electrolysis becomes impractical.
The temperature at which the Perchlorate cell is operated may vary widely, and the temperature of the bath may vary from room temperature up to elevated temperatures as high as about 70C. Preferred practice is to maintain the temperature of the cell between about 25 and about 55C.
As is usual in the electrolythic oxidation of Chlorate to Perchlorate, agitation of the electrolyte bath is desirable and circulation of the electrolyte bath through one or more cells is preferred practice.
The electrolysis of the Chlorate is generally carried out at a pH below about 9, and preferably at a non-alkaline, that is neutral or acid pH. Generally an acid pH is preferred when the cell is to be operated at an elevated temperature.
With respect to the electrical conditions employed, in general current yields improve with increase in current density. In addition, higher anode current densitys tend to minimize the adverse effects of operation at higher temperatures and of competing anode reactions. In the preferred practice of present process, anode current densities of at least 20 ampers per square decimeter (200mA/cm^2), preferably at least about 40A/dM^2 (400mA/cm^2).
The coating of Lead Dioxide on the anode is to be at least 1/16 of an inch thick. The most marked affect of the addition of Persulphate to the cell electrolyte is with Lead Dioxide anodes. However improvements in current efficiency will also be noted with other anode materials.
The cathode can be stainless steel.
The usual Persulphate added will be Potassium Persulphate but any water soluble sulphate can be used. The amount of Persulphate to add may vary somewhat. Amounts in excess of about 10g/l provide no significant improvement over inclusion of lesser amounts, whereas amounts less than about 1g/l do not provide any marked improvement. Electrolysis can be carried out until there is less than 10g/l of Chlorate left.
Example 1
To an aqueous solution containing 606g/l of Sodium Chlorate is added Potassium Persulphate in an amount of 2.08g/l. The resulting solution is electrolyzed using a Lead Dioxide anode and a stainless steel cathode at 300mA/cm^2 at a temperature of 35C. In the Sodium Chlorate concentration range of 606 to 100g/l, the current efficiency is 82.4% and in the range of 606 to 7.1g/l of Sodium Chlorate the current efficiency is 73.3%
Example 2
In a second run employing similar conditions to example 1 the current efficiency is 80.8% in the Sodium Chlorate concentration range of 606 to 100g/l and 68.2% in the range 606 to 30.3 grams per litre Sodium Chlorate
Claims not shown:
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