US2506438A - Electrolytic process for production of fluorine - Google Patents

Description

May 2, 1950 r G. c. WHITAKER 2,506,438

ELECTROLYTIC PROCESS FOR PRODUCTION OF FLUORINE Filed May 14, 1946 Georye 6. WhZa/rer electrolyte.

dismantling and cleaning the; electrolyte cell to Patented May 2, 1950 EIEEGTROLYTIG PROCESSFOR; PRODUCTION OF FLUQRINE;

George C. Whitaker, Brooklyn Heights, Ohio, assigncrto the-Uniteiilv States of America as represented by the United States Atomic Energy Commission ApplicationMay 14, 19,46, Serial,No. 669,5Q2,

2 Claims. (01. 20460) invention relates to an electrolytic apparatus and process for the production of fluorine.

Methods for the electrolytic production of fluorine have. long been known; but, known methods have possessed. disadvantages that prevent large scale. low cost production of fluorine. The. difliculties. are due in large part to the fact that. hydrogen. fluoride, from which the. fluorine is derived by electrolytic decomposition, is s.ub stantially nonrconductiv'e. To render the hydrogen fluoride sufliciently conductive to permit electrolysisi to. take. place, it is. common practice toaddi a. metal fluoride, for example, potassium bifluoride, KHE'Z, since this. latter material is readily available, andnot' hygroscopic. By dissolving the potassium. b'i'fluoride in liquid anhydrous hydrogengfiuori'de an electrolyte is formed "from which hydrogen and fluoride are, evolved upon. passage of an electriccurr'ent therethrough.

The relative proportion. '01" KB to HF in the electrolyte markedly influences the character of the. electrolysis. While the. electrolyte "is most conveniently made up. by use of potassium bifluori'd'e (KHF2 or KF'HF) it is more convenient to express composition in terms of KF and HF as it the electrolyte had been actually made from RF' arrdI-IF. Where the molar ratio is approximately one mole of KF to 8 moles ct HF; the electrolyte is. liquid at room temperatures. With this electrolyte, it has been necessary to employ an anodeof nickel which is rapidly eaten away and forms a fluoridesludge, in thebottom'of the The cost of nickel anodes and. of

remove the sludge makes the process expensive.

Efiorts lac-employ an inert material such as carbon or graphite with such abath. has. proved unsatisfactory due, to, the tendency of such anodes to disintegrate. That is after a shortperiod of operation theanodes tend to swell and "break and the current ceases. to flow.

It is an object of the presentinventionto provide an improved electrolytic process for production of fluorine wherein a high current densitymay be obtained at relatively low voltage and without the use of anodes of nickel or other rapidly consumed expensive material. It is a further object to provide an electrolytic system for the production of fluorine gas by the electrolytic decomposition of hydrogen fluoride which is operable at high current efliciency and high current density.

Applicant has discovered that these desirable results may be obtained by electrolysis of baths including hydrogen fluoride and a plurality of fluorides of univalent metals between a copperfilled carbon anode and a suitable cathode. "In the preferred embodiment of the invention-the system will include an electrolyte comprising potassium fluoride, hydrogen fluoride and lithium fluoride in the proportions of 60% potassium fluoride, 38 /2% hydrogen fluoride and 1 lithium fluoride.

The drawings annexed hereto and constituting a part of the disclosure show a suitable electrolytic cell for use in connection with the instant invention. It will be understood; however, that other cell structure suitable for electrolyticproduction of'corrosive gases maybe used.

Figure 1 is a perspective view partly broken away of a cell usable in connection with the instant invention. Figure '2 is a vertical sectional view of the electroyltic cell taken on the line 2--2 of Figure 1.

With particular reference to Figure l of the drawing, the numeral 4 designates an electrolyte container of generally rectangular form constructed of steel or other suitable material of Y construction and having temperature control jackets 2 on opposite sides'the'reof.

The container 1 is provided with a top '3 of similar material having a rectangular steel ''or Monel metal skirt 4 extending downward-within the container I and forminga separate chamber for collecting fluorine.

Within the cell there are a pair of cathode plates 5 and '6 which may be steel sheets brazed to copper bars 1' and 8 suspended from "electrical connectors 9 and l0. Both of the cathodes 5 and S are located outside of the chamber-formed by skirt 4 and the connectors 9 and It pass up between this skirt and the side walls or the: container l.

Between the two cathodes 5 and 6 a carbon anode l'l is'suspen'ded from electrical connectors l2 which extend upward through the chamber formed by skirt 4.

The-electrical connectors 9, Ill and liareelectrically insulated from the top'3 or the-cell; "The connectors l2 are connected with positive current source (not shown).

Extending downward from the long sides of skirt 4 are a pair of diaphragms l5 and I6 composed of Monel metal screen or preferably perforated Monel metal sheets. If desired, the diaphragm can be continued around the ends of the anodes in the same manner as skirt 4. This provides somewhat greater rigidity for the diaphragm.

A gas outlet conduit I! for removal of the fluorine product is provided in top 3 and communicates with the anode chamber formed by skirt 4. Outlet conduit l8 for removal of hydrogen is provided in top 3 and communicates with the cathode chamber.

For the addition of hydrogen fluoride to the cell, a pair of pipes l9 and 20 each connected to a serrated distributer 2! are provided.

In accordance with the instant invention, the anodes are copper-filled carbon. That is, anodes containing 35 to 47% of copper in the form of network distributed therethrough. These anodes may be suitably formed by vacuum impregnation of porous carbon with molten copper. The anodes should be made of substantially pure carbon, suitably such as may be derived from petroleum coke. The pure carbon may be mixed with pitch and fired at a temperature just below that at which graphitization occurs. These, when impregnated with copper, have given best results in my work. The electrolyte employed according to a, preferred embodiment comprises from 78 to 87 parts by weight of potassium bifluoride -(KHF'2) 13 to 22 parts by weight of hydrogen fluoride (liquid anhydrous HF) and from 0.1 to

between 1.6 to 1 and about 2.1 to 1.

The electrolyte formed as above described melts at a temperature below 100 C. and has been found highly suitable for electrolysis in the in-- stant system between a cathode of copper or iron and an anode of copper filled carbon.

In operation of this cell to produce fluorine, the cell is filled to a point above the bottom of skirt 4 with a suitable electrolyte such as a potassium acid fluoride having the composition above described. Because of the formation of gas bubbles in the electrolyte, the electrolysis causes the liquid level to rise to the operating level L.

The cell is brought to operating temperature, about 100-110 C., and current is applied by the cathodes 5 and 6 and the anodes H and the cell is operated with a current flow between these anodes and the cathodes 5 and 6.

During operation of the cell the operating temperature is maintained by passing cooling fluid through the jackets 2, and the hydrogen fluoride content of the electrolyte is maintained by continuously or intermittently introducing makeup hydrogen fluoride through inlets l9 and 20. The gases produced by the cell are withdrawn through outlets l1 and I8.

-As an example of the operation of the device in a cell constructed as described above wherein the anodes and the cathodes were spaced a distance of 1%.; inches, it was found that fluorine was steadily produced with a current density of 90 amperes per square foot at a temperature of 110 C. The cell operated for a period of 2,530 hours without trouble from polarization from the anodeand acurrent efficiency of 90 to 95%.

While the invention has been described in its operation between an iron cathode and a copper filled carbon rod, and electrolyte comprising potassium bifluoride, hydrogen fluoride and lithium fluoride in the defined proportions, it will be understood that variations may be made within the spirit of the invention. Accordingly, it is to be undersoood that the invention is not to be construed as limited to the specific conditions and details described but is intended to include all processes within the scope of the appended claims.

I claim:

1. A process for the electrolytic production of elemental fluorine comprising passing an electric current through an electrolyte between a steel cathode and an anode composed of carbon containing from -about 35 to about 47% copper in the form of a network distributed therethrough, the electrolyte consisting essentially of hydrogen fluoride, potassium fluoride and lithium fluoride, the molar ratio of hydrogen fluoride to potassium fluoride being between about 1.6 to 1 and about 2.1 to 1 and the lithium fluoride being present in the amount between about 0.1% and about 2.5% by weight of the total electrolyte, the electrolysis being carried out at a'temperature of from about to about C.

2. A process for the electrolytic production of elemental fluorine comprising passing an electric current through an electrolyte between an iron cathode and an anode composed of carbon containing from about 35 to about 47% copper in the form of a network distributed therethrough, the current density being from 40 .to amperes per square foot and the electrolyte consisting essentially of hydrogen fluoride, potassium fluoride and lithium fluoride, the molar ratio of hydrogen fluoride to potassium fluoride being between about 1.6 to 1 and about 2.1 to 1 and lithium fluoride being present in amount between 0.1% and 2.5% by weight of the total electrolyte, the electrolysis being carried out at a temperature of from about 95 to about 115 C.

GEORGE C. WHITAKER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,484,734 Mathers Feb. 26, 1924 1,888,118 Fredenhagen Nov. 15, 1932 2,325,201 Young July 27, 1943 2,422,590 Schumb et al June 17, 1947 OTHER.- REFERENCES Transactions of The Electrochemical Society, vol. 66, (1934), pa es 245-252.

Chemical and Metallurgical Engineering, July 1946, pages 106-108.

Industrial and Engineering Chemistry, vol. 39 (1947), pages 255-258.

Claims (1)

1. A PROCESS FOR THE ELECTROLYTIC PRODUCTION OF ELEMENTAL FLUORINE COMPRISING PASSING AN ELECTRIC CURRENT THROUGH AN ELECTROLYTE BETWEEN A STEEL CATHODE AND AN ANODE COMPOSED OF CARBON CONTAINING FROM ABOUT 35 TO ABOUT 47% COPPER IN THE FORM OF A NETWORK DISTRIBUTED THERETHROUGH, THE ELECTROLYTE CONSISTING ESSENTIALLY OF HYDROGEN FLUORIDE, POTASSIUM FLUORIDE AND LITHIUM FLUORIDE, THE MOLAR RATIO OF HYDROGEN FLUORIDE TO POTASSIUM FLUORIDE BEING BETWEEN ABOUT 1.6 TO 1 AND ABOUT 2.1 AND THE LITHIUM FLUORIDE BEING PRESENT IN THE AMOUNT BETWEEN ABOUT 0.1% AND ABOUT 2.5% BY WEIGHT OF THE TOTAL ELECTROLYTE, THE ELECTROLYSIS BEING CARRIED OUT AT A TEMPERATURE OF FROM ABOUT 95 TO ABOUT 115*C.

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