US2985569A - Electrolytic method and means for production of refractory metal - Google Patents

Description

May 23,1961 L. w. GENDVIL 2,935,569

ELECTROLYTIC METHOD AND MEANS FOR PRODUCTION OF REFRACTORY METAL Filed July so, 1956 Zr Cl Fig. I.

Fig. 2.

INVENTOR Leonard W. Gendvil AGENT ELECTROLYTIC METHOD AND MEANS FOR PRO- DUCTiON F REFRACTORY METAL Leonard W. Gendvil, Dongan Hills, Staten Island, N.Y., assignor to National Lead Company, New York, N .Y., a corporation of New Jersey Filed July 30, 1956, Ser. No. 600,758

5 Claims. (Cl. 204-64) This invention relates to the electrolytic production of refractory metals. More particularly, it relates to the production of high purity zirconium metal by an electrolytic process adapted specifically to the production of zirconium metal on a commercial scale.

There are many compounds of refractory metals which are found in nature but which are extremely difiicult to reduce to the metallic state. In many instances it is relatively easy to convert such compounds to the halides of the refractory metals by halogenation processes, but it is ditficult to produce the refractory metals from their respective'halides. Included in the group of metals which fall within the class known as refractory metals are titanium, zirconium, vanadium, niobium, tantalum, molybdenum, tungsten, thorium and uranium.

Some of these refractory metals have heretofore been produced from their compounds by thermal reduction methods employing pressure and a reducing metal, while others have been produced by direct chemical reduction of the halide employing metallic sodium or metallic magnesium as reducing agents. While these methods have produced such metals, they suffer from a fundamental economic disadvantage in that the cost of-the reducing metal employed, such as sodium or magnesium, is relatively high and the processes are expensive and cumber-v some to operate.

The production of titanium metal by a direct electrolytic process has now been accomplished, but despite the fact that titanium and zirconium metals are each in group IVa of the periodic table attempts to produce zirconium by such process have not been successful. The

instant application relates to an improved direct electroly- A still further object of the invention is to provide a superior electrolytic cell for producing highly ductile nonpyrophoric zirconium metal on a commercial scale.

These and other objects of this invention will become apparent from the following more complete description thereof and the accompanying drawings in which:

Fig. 1 is a vertical cross section'of an electrolytic cell embodying a cathode in the form of a conical metal member; and

Fig. 2 is a plan view of the cellon section line of Fig. 1.

In its broadest aspects, the instant invention contemplates a semi-continuous method for producing a refrac- 2,985,569 Patented May 23, 1961 ice tory metal, and in particular zirconium metal, in an-elec-,

trolytic cell having a cathode, an anode and a fused salt bath by feeding vaporous ZrCl into said bath below the surface thereof an in immediate proximity to a cathodic surface while simultaneously passing direct current between the anode and said cathodic surface at a rate synchronized with theZrCl, addition so that the .amount ofcurrent is sutficient to reduce the ZrCl being added to zirconium metal; and depositing the zirconium metal asan adherent mass of-coarse crystalline metal on the cathodic; surface while maintaining the bath substantially free of reduced zirconium values except in the immediate vicinity of the cathodic surface. g v

More particularly, the invention contemplates the do position of. a coarse crystalline deposit of non-pyrophoric zirconium metal on the interior surfaces of 'a bell-shaped,

cathode having a perforated bottom by introducing the. vaporous ZrCl into the interior of the bell-shaped cath-- ode below the surface of the electrolyte, and maintaining the adherent mass of zirconium metal and the perforated cathode perforative and the bath exteriorly of the bellshaped cathode substantially free of reduced zirconium values.

As has been disclosed in the electrolytic art relating to the production of titanium, the halides of titanium and'in particular the tetravalent chlorides and bromides of titanium, while not readily soluble in a fused salt bath, nevertheless can be electrolyticallyreduced to titanium metal by passing direct current through the cell at a rate synchronized with the rate of titanium metal'halide addi-1 tion to the fused salt bath so that the amount of currentis sufficient to reduce a substantial portion, if not all of the titanium metal halide, substantially directly to titanium metal. This technique is ,hereinafter referred to as sub-. stantially direct electrolysis of titanium metal halides in a fused salt'bath. i

According to the present invention, it'has been discov-. ered that zirconium metal may also be produced byv direct electrolysis of zirconium halides and that recoveries of a coarse crystalline. non-pyrophoric metal on a commerically practical scale are obtained by c'onfiningthe zirconium metal halide in a fused salt bath to'the immediate vicinity of a cathodic surface such that the zirconium metal will be deposited substantially in toto thereon; and that the production of'this superior form of Zr metal is effected by controlling the operating conditions of a particular cell and the type and orientation of the cathodic surface relative to the anode and the point of introduc tion of the zirconium halide into the fused salt bath such that thezirconium metal deposit is maintained perfora tive throughout a given run.

The phrase confined to the immediate vicinity of the cathode, as used herein, will be understood to mean that the zirconium halide introduced into the fused salt bath" is not permitted to diffuse throughout the molten salt electrolyte but is directed toward and maintained substantially in intimate contact with a cathodic surface whereby the zirconium halide values are reduced to metal in the form of a zirconium metal deposit on the cathodic 'sur-T face. The preferred method and means by which the zirconium halide may be confined to the immediate vicinity of a cathodic surface and the metal deposit formed thereon forms the subject matter of the instant invention and is described hereinafter in detail.

The phrase operating conditions, as used herein, has reference to the control which must be maintained over variables such as time, temperature of the fused salt bath, the cathode current density, current-to-ZrCl, feed ratio, cathode design, and zirconium content of the elec trolyte within the cathode in order to carry out the step.- of maintaining the deposit of zirconium metal'in perfora-: tive form.

In this connection it should be explained that by perforative is means a porous crystalline type of deposit as distinguished from a dense solid deposit. While there is some doubt as to the exact nature ofthe electrolytic chemistry involved, it has been found that direct electrolytic reduction of zirconium halide values to zirconium metal as large deposits of ductile non-pyrophoric crystalline metal can be accomplished on a commercial scale by the use of a cathodic surface, preferably in the form of a metal cone, having a grill or screen across its open base, into which the ZrCl is introduced below the surface of the electrolyte. It may be postulated that ionic currents proceed from the anode through the screen in the bottom of the cathodic cone and through the voids in the perforative deposit of zirconium metal to the inside of the cone where zirconium ions maybe found andreduced to zirconium metal on the inner walls of the cone. Contrary to what one might expect, a major portion of the ionic current does notstop at the exterior surfaces of the cathodic cone but apparently continues on through the cathodic screen base of the cone and through the relatively infinite number of substantially parallel paths in the perforative deposit of zirconium metal to the in terior of the cone,

In order to describe more clearly the details of the instant invention, the process will be specifically illustrated by describing the production of zirconium metal from zirconium tetrahalide.

. Referring to Fig. 1, the apparatus shown consists of a cell container heated by graphite electrodes 11,11. The cell container is filled or partially filled with an electrolyte 12 in which is suspended a pair of graphite anodes 1313 between which is supported a cathodemember indicated generally at '15. The fused salt electrolyte, sometimes referred to hereinafter as a fused salt bath, comprises preferably a molten-halide salt of an alkali or alkaline earth metal, including magnesium, particularly the chlorides of said metals which may be employed singly or in combinations. Mixtures of these halides whichform low melting point eutectics are most convenient to employ such as, for example, mixtures of sodium. chloride and strontium chloride, sodium chloride and lithium chloride, sodium chloride and barium chlo ride, sodium chloride and magnesium chloride or mixtures thereof. In general, the temperature of thefused salt bath may vary within the range of from 600 C. to 950 C. depending upon the particular salts used, the type of metal being deposited and the construction of the cell itself. 'For the production of zirconium metal using a. sodium chloride bath, operating temperatures within the range of from 750"-850 C. have been found preferable. w

Referring again to the cathode member 15, the latter comprises a feed pipe 16 which is used for introducing the tetrachloride into the electrolyte and which extends coaxially of a metal-pipe '17'connectedat its upper end to the negative side of a current source; and having a bell-type enlargement at its lower end comprising preferably a metal cone 18 provided with a screen or grid 19 across its open base. The cone may be formed of iron or nickel, and inasmuch as it is integral with or connected electrically to the cathodic metal pipe 17, it itself is cathodic. An important control in the operation of thecell is that 'of introducing the incoming zirconium chlorides into the fused salt bath at a point in the cathodic cone 18 below its apex, and to this end the feed pipe 16 extends down into the cone far enough so that the lower extremity of the pipe terminates close to the screen base of the cone. As a consequence, the zirco nium chlorides are brought into intimate contact with the cathodic surfaces which confine andrestrict the dispersion of the zirconium chlorides to thatportion of the electrolytewithin the interior of the cone such that zirconium metal is deposited on the'bottom and walls thereof in the form of-relatively-large particles of ductile met It is significant that whereas substantially cylindrical perforated metal baskets (iron or titanium metal) have been used successfully to produce coarse crystalline deposits of titanium metal, it has been found that for the production of Zr metal the use of a bell-type member having a screen over its open bottom end is a definite aid in the deposition of crystalline non-pyrophoric zirconium metal.

The screen or grid base 18. of the cathodic cone is also iron or nickel and is a relatively coarse screen, the apertures of Which may. vary from 8 to 256 per sq. inch area of the screen, a preferred screen size being apertures to the square inch. In this connection the overall area of the screen 18 is the area. used in computing the cathode current density which may vary from 200 to 800 amperes per square foot;

For reduction to take place on the inside or inner surfaces of the zirconium deposit, it is necessary that the ionic current pass through the perforative deposit of zirconium metal to the interior of the cone where the zirconium ions .are found. Asfthe deposit increases in thickness, it offers increased resistance to the passage of the ionic current which for, constant rate of reduction requires higher voltage and amperage. This in turn results in lower current efficiencies. In brief, the nature of the operation'is such that it will cut itself off. However, if operated under proper conditions a deposit of practical thickness and porosity will. be formed on the inner walls of the cone.

In carrying out the process of the instant invention, the metal halide is added, preferably in vapor form, to the fused salt bath concurrently with the addition of current. In order that the halide may be added at a substantially constant rate, it is preferred to meter the halide in the vapor state.

As mentioned above, in order to reduce zirconium tetrachloride to metal, apredetermined amount of current must be passed concurrently through said cell at a rate synchronized with the rate of zirconium tetrachloride addition. A theoretically sufiicient current will comprise about four faradays of electricity passed concurrently through the cell while approximately one mol of zirconium tetrachloride is being introduced into the cell. In actual practice, however, it has been found desirable to add a quantity of electricity somewhat in excess of the theoretical amount in order to make up the current loss caused by said reactions in the cell and to account for variations in the rate of feed of the zirconium tetrachloride. While in theory it would be sufiicient to add about 4-5 faradays of electricity to maintain efficient cell operation, in practice variations in sublimation rate, and hence the feed rate of vvaporous zirconium tetrachloride may necessitate modification of the theoretical current-to-feed ratio such that five to fifteen. faradays of electricity are required per mole of zirconiurntetrachloride.

An alternative expediency isto vary the current to fit the ZrCl feed rate and to use the cell as an indication of the. cur-rent-to-feed ratio that prevails in the system at any, given instant. For a cathode having a desirably perforative deposit a cell of 2.3-2.6 volts will be indicative of a current-to-feed ratio of about 5.6 faradays/mole while an E.M.F. of 2.72.8 is indicative of a current-to-feed ratio of 8-12 faradays per mole. Moreover, it is known that the cell is a function of the concentration of reduced chlorides in the melt, and that a low E.M.'F., i.e. 2.3-2.6 volts, is indicative of high concentrations of reducedchlorides in the melt while a high E.M.F., i.e. 2.72.8-volts, is indicative of low concentrations of reduced chlorides in the melt. The higher concentrations corresponding to low tend to produce a massivedeposit of zirconium whereas the lower concentrations of reduced zirconium-chlorides produce a dendritic deposit.

' In order to illustrate the invention further the following examples are given,

Example I Using a cell similar to that shown in Figure l a conical nickel cathode having a nickel screen over its open basewith A apertures spaced 3& on centers and having a nickel feed pipe for the introduction of zirconium tetrachloride vapors into the interior of the cathodic cone ad- .jacent the bottom thereof was lowered into a fused salt electrolyte consisting of 400 pounds of sodium chloride proximately 6 volts was required. The cathode currentdensity was about 500 amperes per square foot. The run was made for a period of 6 hours during which time the deposit of zirconium metal remained perforative and the apertures in the screen open. At the end of this period the introduction of zirconium tetrachloride vapor was stopped and no further current was passed through the electrolyte. The cathodic cone was withdrawn from the cell and zirconium metal was found deposited on the walls of the cathode in the form of an irregular perforative tenacious mass composed of relatively large crystals. The zirconium metal was removed from the cell and cooled in a chamber having an inert atmosphere. The cooled deposit was leached and the dried leached zirconium metal was recovered as coarse crystals weighing 344 grams having a definite metallic luster and being quite ductile. A sample prepared by are melting these crystals possessed a Brinell hardness of 198. About 87% of the zirconium values introduced as zirconium tetrachloride were recov- 'ered as zirconium metal. The cell was 2.4 volts.

Example II Using a cell similar to that shown in Fig. 1, a conical nickel cathode provided with a nickel screen over its base having apertures spaced on centers and having a nickel feed pipe for introducing zirconium tetrachloride vapors into the cathode was lowered into a fused salt electrolyte consisting of 400 lbs. of sodium chloride maintained at a temperature of about 900 C. Zirconium tetrachloride vapors were introduced into the interior of the cathode below the surface of the electrolyte at the rate of 95 grams per hour and zirconium metal was deposited on .the walls of the basket as relatively large coarse particles of metal. Concurrently an electric current equivalent to 9.3 faradays per mole of zirconium tetrachloride was passed through the cell. This amount of current was in effect sufiicient to completely reduce the zirconium tetrachloride to zirconium metal. In order to obtain substantially 9.3 faradays per mole of zirconium tetrachloride introduced, 100 amperes with an impressed voltage of approximately 5.6 volts was required. The cathode current density was about 400 amperes per square foot. The run was made for a period of 6 hours during which time the deposit of zirconium metal remained perforative and the apertures in the screen open. The cathode was withdrawn from the cell and zirconium metal was found deposited on the interior walls of the cone in the form of an irregular perforative tenacious mass composed of relatively large crystals. The leached zirconium metal recovered, as described in the preceding example, weighed 148 grams which analyzed substantially 99.8% zirconium and possessed a Brinell hardness of about 222. The cell was 2.8 volts.

Example 111 Using the same technique described in Examples I and II with a salt bath comprising 400 pounds of 30 mole percent KCl, 34 mole percent Srclg and 36 m'ole" percent NaCl, and a cathodic iron cone having a-nickel screen with perforations spaced 256 holes to the square inch and a feed pipe, ZrC l was fed into the electrolyteat the rate of grams perhour, the temperature of the bath being 700 C. Electric current equivalent to 7.2' faradays per mole of ZrCl was passed through the cell,"

the amount of current required being IOOamperes at an impressed voltage of about 6 volts. The cathode current density was about 400 amperes per square fo'ot'and the run was continued uninterruptedly for 7.3 hours. The

coarse zirconium metal produced by this runweighed was about 342.

approximately 200 grams, the Brinell hardness of'which It has been clearly shown by the description'cf the instant invention and by the examples presented 'tha't refractory metals may be obtained by passing a refractory" metal halide into an electrolytic cell by means of a coni-g cal nickel cathode at a rate synchronized with the elec-* tric current addition such that the amount-of electricity added per mole of refractory metal halide measured infaradays is numerically substantially greate'rf thanthe number of halide atoms present in the said refractory metal halide molecule; and that-by confining thereduced halides to the immediate'vicinity of the cathode while concurrently retaining the zirconium metal deposit on the cathodic surfaces, high recoveries of ductile relatively coarse zirconium metal may be obtained. Thus; the: electrolytic process of the instant invention employs sim-' ple and inexpensive apparatus whereby it is' possible' to produce refractory metals economically and continuously."

While this invention has been described and illustrated by the examples shown, it is not intended to be strictly limited thereto, and other modifications and variations may be employed within the scope of the following claims.

I claim:

1. A method for producing zirconium metal in an electrolytic cell having an anode, bell-type cathode having a perforated bottom, and a fused salt bath consisting essentially of salt selected from the group consisting of alkali metal halides, alkaline earth metal halides, magnesium halides and mixtures thereof comprising the steps of: introducing ZrCl below the surface of said bath into the interior of said bell-type cathode, passing electric current between the anode and said bell-type cathode at a rate synchronized with the ZrCl addition so that the amount of current is sufiicient to reduce the ZrCl being added to metal and depositing an adherent mass of zirconium metal in the interior of the bell-type cathode, while maintaining the electrolyte exteriorly of the cathode substantially free of reduced zirconium values.

2. A method for producing zirconium metal in an electrolytic cell having an anode and a fused salt bath consisting essentially of salt selected from the group consisting of alkali metal halides, alkaline earth metal halides, magnesium halides and mixtures thereof comprising the steps of: submerging a bell-type cathode having a perforated bottom in said fused bath, introducing ZrCl below the surface of said bath into the interior of said bell-type cathode, passing electric current between the anode and cathode at a rate synchronized with the ZrCl; addition so that the amount of current is suflicient to reduce the ZrCl being added to metal, depositing zirconium metal in the interior of the cathode, meanwhile maintaining the bottom of said cathode and the deposit of zirconium metal in perforative form and maintaining the electrolyte exteriorly of the cathode substantially free of reduced zirconium values.

3. A method for producing zirconium metal in an electrolytic cell having an anode and a fused salt bath consisting essentially of salt selected from the group consisting of alkali metal halides, alkaline earth metal halides, magnesium halides and mixtures thereof comprising the steps of: submerging a bell-type cathode having a persimultaneously controlling the operating conditions of a said cell such that said adherent mass of zirconium metal is perforative andthe eleqtrolyte exteriorly of the cathode is substantiallyfiree t reduced zirconium values.

4. A method for producing zirconium metal in an electrolytic cell having an anode and a fused salt bath consisting essentially of salt selected from the group consisting of alkali metal halides, alkaline earth metal halides,

magnesium halides and mixtures thereof comprising the' steps of: submerging a coneshaped cathode having a perforated bottom in said fused bath, introducing ZrCl below the surface of said bath into the interior of said cathodic cone, passing electriccurrent between the anode and cathodic cone at a rate synchronized with the ZrClA, addition so that the amount of current is sufiicient to reduce the ZrCl being added to metal, depositing an adherent mass of zirconium metal in the interior of the cathodic cone, and maintainingthe current-to-feed ratio withintthe range of from 5-15 faradays per mole and the cathodecurrent density within the range of from 200- 800 amperes per square foot suchthat said adherent; mass of zirconium is perforative and the electrolyte exteriorly oi the cathodic cone is substantially free of reduced zirconium values.

Cir

5. A method for producing zirconium metal in an electrolytic cell having an anode and a fused salt bath consisting essentially of salt selected from the group consisting of alkali metal halides, alkaline" earth metal halides, magnesium halides and mixtures thereof comprising the steps of: ,submerging a cone-shaped cathode having a screen over its open bottom end in said fused bath, intro: ducing ZrCl into the interior of said cathodic cone adjacent the screen at the bottom thereof, passing electric current between the anode and cathodic cone at a rate synchronized with the ZrCL, addition such that the cell is in the range of from 2.7-2.8 volts and the oathode current density is within the range of from 400-v l 500 amperes per square foot, depositing an adherent mass of zirconium metal in the interior of the cathodic cone,

and maintaining said adherent mass of zirconium metal in perforative form and the electrolyte exteriorly of the cathodic cone substantially free of reduced zirconium values.

References Cited in the file. of this patent UNITED STATES PATENTS Opie et al. Aug. 6, 1957

Claims (1)

1. A METHOD FOR PRODUCING ZIRCONIUM METAL IN AN ELECTROLYTIC CELL HAVING AN ANODE, BELL-TYPE CATHODE HAVING A PERFORATED BOTTOM, AND A FUSED SALT BATH CONSISTING ESSENTIALLY OF SALT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL HALIDES, ALKALINE EARTH METAL HALIDES, MAGNESIUM HALIDES AND MIXTURES THEREOF COMPRISING THE STEPS OF: INTRODUCING ZRCL4 BELOW THE SURFACE OF SAID BATH INTO THE INTERIOR OF SAID BELL-TYPE CATHODE, PASSING ELECTRIC CURRENT BETWEEN THE ANODE AND SAID BELL-TYPE CATHODE AT A RATE SYNCHRONIZED WITH THE ZRCL4 ADDITION SO THAT THE AMOUNT OF CURRENT IS SUFFICIENT TO REDUCE THE ZRCL4 BEING ADDED TO METAL AND DEPOSITING AN ADHERENT MASS OF ZIRCONIUM METAL IN THE INTERIOR OF THE BELL-TYPE CATHODE, WHILE MAINTAINING THE ELECTROLYTE EXTERIORLY OF THE CATHODE SUBSTANTIALLY FREE OF REDUCED ZIRCONIUM VALUES.

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