US3503858A - Continuous electrolytic cell process - Google Patents

Description

March 31, 1970 a. J. cams CONTINUOUS ELECTROLYTIC CELL PROCESS Filed April 5, 1965 2 Sheets-Sheet 1 FIG. I

United States Patent Int. Cl. C01b 11 /18; B01k 1/00 US. Cl. 204-82 3 Claims ABSTRACT OF THE DISCLOSURE The operation of multipolar electrolytic cells in the production of an alkali metal chlorate or perchlorate, particularly sodium chlorate or perchlorate, by the electrolysis of an electrolyte comprising an aqueous solution of an alkali metal chloride on a continuous basis. In the continuous operation, the electrolyte is continuously recirculated through the cell from a cell tank to effect electrolysis, a small proportion of the recirculating volume of the electrolyte being continuously withdrawn from the cell tank and fresh electrolyte being continuously fed to the cell tank. By the improved operation of the multipolar cell according to the present invention, the overall efficiency of the process for the production of the alkali metal chlorate or perchlorate is substantially increased.

This invention relates to a new and improved process and apparatus for the electrolysis of brine and more particularly for the continuous electrolysis of an alkali halide brine to alkali metal chlorates and perchlorates carried out in multipolar electrolytic cells.

The most economic manufacture of sodium chlorate and perchlorate is usually performed in multipolar electrolytic cells generally installed in a large tank. These cells comprises a series of parallel electrodes mounted in a box and sealed at their edges and bottom to prevent leakage of solution or electric current, the solution to be electroylsed being maintained between each set of electrodes. A brine such as NaCl is electrolysed in the cells through a number of intermediate reactions to form the sodium chlorate, with hydrogen gas evolved as a by-product when oxygen from the water is freed to enter into the reaction. The overall chemical reaction is represented by:

The brine solution containing NaCl and some NaClO circulates between the'electrodes and at each pass some of the NaCl is converted to NaClO Fresh solution enters each electrode compartment at the lower edge of the electrode through an inlet pipe, and the natural circulation of the liquor between the electrodes is caused by the generation of hydrogen gas between the cell plates. This reduces the average specific gravity of the material between the electrodes. The heavier dense fresh liquor from the tank enters into the bottom inlet pipes and forces out the mixture of liquid and gas through upper pipes positioned at the top of the electrodes. In addition, a minor circulation force is caused by a temperature increase of the electrolyte in the cells between the electrodes. A number of these large tanks will be located in a chlorate plant and all of these tanks will be operated on a batch principal. The usual output from a single tank varies from between 2000 and 2000 tons per year depending on its size.

Operating a large tank as a batch process has many disadvantages. For example, the electrolyte in each tank must be batch operated and separately analysed and controlled by adding various materials such as water, brine, acid, chromate, a soluble calcium, salt, etc., when they be- 3,503,858 Patented Mar. 31, 1970 come depleted in the electrolyte. Furthermore, as the electrolyte becomes enriched with the desired components, the voltage necessary for maintaining the current flow through the cell is increased. As an example, a freshly filled tank would require only volts for a full production rate, but the same tank when the electrolysing is finishing would require about volts. Since this variation takes place across each cell tank, the voltage supply must be variable over a relatively large range. This increases the capital cost of equipment and increases the peak demand for electrical energy; hence the cost per kilowatt hour of electric power will increase.

In addition more electric power is required for a greater final concentration of NaClO In the usual plant a final concentration of between 450 g.p.l. and 9 00 g.p.l. of NaClO is necessary so that the NaClO crystals can be economically extracted from their mother liquor.

Large storage containers are required if a tank is to be quickly filled after it has been emptied, and in addition other large storage containers are required to receive the electrolysed product when it is removed from a cell.

During the emptying of a cell tank after completion of a process run, the entire facility on the same circuit must be disconnected. Since emptying and filling a tank required approximately 1 to 3 hours, a typical plant of 4 to 8 cell tanks on one circuit will incur a lost production time offrom 5% to 15%.

The use of batch operations further requires the need of a high capacity pump to quickly fill and empty these tanks. Also operating time is lost on commencing a new electrolysis run since the temperature must be allowed to come to equilibrium operating conditions. Finally the sudden temperature changes which are produced on the tank/cell box will adversely affect the component parts after sufficient use.

It is an object of this invention to overcome the drawbacks of the prior art and to provide an increase in productivity in the order of 5%.

Further objects of the invention will become apparent from the description to follow.

The objects of the invention are attained in an apparatus comprising a multipolar cell contained in a cell tank, means for supplying fresh feed to said cell tank, means for internally recirculating electrolysed liquor within said multipolar cell and means for withdrawing enriched electrolysed liquor from said multipolar cell.

In the drawings:

FIG. 1 represents a perspective View of a multipolar electrolytic cell; and

FIG. 2 represents a perspective view of the cell embodying the recirculation system according to this invention.

In the diagrams, an electrolytic multipolar cell tank is shown having side walls 1, monopolar electrodes 2 and multipolar electrodes 3 of graphite, or other conducting material. The multipolar cell is contained in a cell tank 5 containing an alkali earth' metal brine such as sodium chloride, the multipolar cell being immersed to level 7. Since the electrolyte in the multipolar cell is at a level 6, the hydrostatic difference in the two levels allows the electrolyte to flow out through upper outlet tubes 10 while liquor from cell tank 5 enters the multipolar cell through inlet tubes 4.

According to this invention, the outflow through tubes 10 is internally recirculated from the electrolytic cell 1 to the cell tank 5. This may conveniently be done by passing the outflow through the open ends of outflow troughs 8, then through the larger body of liquor in the cell tank 5 where it is cooled by coming. in contact with cooling coils (not shown) and back through the inlet tubes 4. This will be continued until a suitable concentration of NaCl and NaClO is produced. As shown in FIG.

2, pump P removesla very small proportionofliquor from the collecting trough or if desired (but not shown) from cell tank 5 into storage or for further processing, etc. Fresh NaCl brine is added to the main liquor body of the tank through inlet tubes 9 to adjust the liquor to the desired concentration of NaCl and NaClO When the desired concentration is reached some of the liquor is pumped from the outflow trough to processing equipment where hypochlorites, chromates, graphite or other deleterious matter is removed. Nothing in this invention limits the feed of impure solution containing all or a part of these impurities directly into the processing equipment. Chromates contaminants can be added for instance to a chlorine dioxide generator without harm. A final adjustment to the liquor requiring addition of a small amount of NaCl as brine or crystals or of NaClO crystal may be required for precise adjustment of the product. According to this invention only a part of the liquid is drawn 011 through outflow trough 8. Fresh treated brine enters the bottom of the tank 1 compensating in volume for that leaving through the outflow trough. Any desired concentration of NaCl or NaClO can be obtained direct ly from the cell tank as within the following limits:

(a) a product containing NaCl at saturation and a trace of NaClO (b) a product containing 40 g.p.l. NaCl and 900 g.p.l.

NaClO In particular the specific feed for chlorine dioxide generators can be made'most economically.

In the present invention the final concentration of NaClO is in most cases only about 300 g.p.l. NaClO versus a final concentration of 450-900 g.p.l. for the usual processes. Consequently the electric power saving in the present invention for direct feed to a chlorine dioxide generator will be between 3 and 8%.

I claim: I

1. In the production of a compound selected from an alkali metal chlorate and an alkali metal perchlorate which includes electrolysing an electrolyte comprising an aqueous solution of an alkali metal chloride in a multipolar electrolytic cell comprising a plurality of spaced bipolar electrode plates forming a plurality of separate individual cell spaces, opposite surfaces of each bipolar electrode being of opposite polarity in respect of the electrolyte in which they are immersed, the current flow being normal to said electrolysing surfaces and the electric potential in any one of the spaces between the electrodes being different from that in the next adjacent space across the cell, in which process the electrolyte is continuously recirculated upwardly through said cell during electrolysis from a cell tank surrounding said cell to effect miXing of the electrolyzed electrolyte from each of said cell spaces only in said cell tank for equalization of electric potential and completion of reaction of the electrolysis products to produce said chlorate or perchlorate, the improvement which comprises continuously withdrawing a small proportion per hourof the volume of electrolyte recirculating in said cell tank and continuously introducing fresh electrolyte into said cell tank whereby to effect the process on a continuous basis.

2. The process as claimed in claim 2 in which the alkali metal is sodium.

3. The process as claimed in claim 1 in which said small proportion is up to 10% by volume per hour of the recirculating electrolyte in a cell tank.

References Cited UNITED STATES PATENTS JOHN H. MACK, Primary Examiner D. R. I ORDAN, Assistant Examiner US. Cl. X.R.

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