US2935454A - Method of the electrodeposition of titanium metal - Google Patents

Description

May 3, 1960 SHIN-ICHI TOKUMOTO 2,935,454 METHOD OF THE ELECTRODEPOSITION OF TI'fANIUM METAL Filed. 001;- 28. 1958 AMP v 0 0m -7'/ME TIME T: .14:. Tr ab.

TIME INVENTOR. SHIN-ICH/ TO/(UMOTO A TTORNEYZ United States Patent METHOD OF THE ELECTRODEPOSITION 0F TITANIUM METAL Shin-Ichi Tokumoto, Tokyo, Japan Application October 28, 1958, Serial No. 770,066 Claims priority, application Japan May 1, 1953 Claims. (Cl. 204-64) This invention relates to a method of the electrodeposition of titanium metal, and more particularly to a method of using fluctuating electric currents in the eleetrodeposition of titanium metal in a compact form, i.e., in the form of a plate or a cake. The said process is carried out in a fused salt bath which contains simultaneously at least four substances, namely barium bromide, magnesium bromide, sodium bromide and titanium bromides.

Titanium metal has heretofore been electrodeposited only in the form of powder, small pellets or sponge by a fused salt electrolysis method.

Since titanium metal readily reacts with oxygen and nitrogen, aftertreatment of the deposits in the form of powder, pellets or sponge is usuallyaccompanied with many difiiculties. r t

The present invention is a method of electrodepositing titanium metal in compact forms, i.e., in the form of plate or cake; it features the use of fluctuating electric currents, which perform the function of alternately (1) electrodepositing titanium and magnesium metal at the cathode and (2) permitting the deposited magnesium to be released into the electrolytic bath as it reduces titanium halides in the bath. The electrolytic baths consist of fused salts which contain at least the following four substances; barium bromide, magnesium bromide,

' sodium bromide and titanium bromides.

An object of the present invention is to provide a method wherein titanium metal is electrolytically produced from titanium halides. .A further object of this invention is to provide a method for the electrolytic production of titanium metal in compact forms, i.e., in the form of plate or cake. A still further object is to provide a method for the economical production of titanium metal. Other objects and advantages of the present invention will become apparent from the following detailed descriptions of this invention.

In order to enable the invention to be more readily understood, reference is .made to the accompanying drawings which illustrate diagrammatically and by way of example, embodiments thereof and in which:

Fig. 1a is a diagram of an electric circuit embodying the method of the present invention, wherein the electrodes are not connected outside the cell during the time of interruption of the electrolyzing currents;

Fig. 1b is a typical wave form of the current in employing the circuit shown in Fig. 1a;

Fig. 2a is a diagram of a modified electric circuit em bodying the method of the present invention; a

Fig. 2b is a representative wave form of the current in employing the circuit indicated in Fig. 2a;

Fig. 3a is a diagram of a further modified circuit for embodiment of the method of the present invention and Fig. 3b is a representative wave form of the current in employing the circuit indicated in Fig; 3a.

In the present invention, the time during which currents pass through the cathode surface with .a higher current density is alternated with the time during which the current density is lowered, and/or with the period 2,935,454 i atentecl May 3, 1960 titanium, in the form of metal halides: more specifically,

5 it contains simultaneously at least four substances such as barium bromide, magnesium bromide, sodium bromide and titanium bromides. Under the conditions specified above, magnesium metal is deposited onto the cathode surface together with titanium metal, during the time in which the currents pass through the cathode at a higher current density. When the current density is lowered the deposited magnesium metal will act to reduce the titanium halides in the bath, and thus return into the bath as magnesium halides.

In the absence of the above mentioned behavior of magnesium, it is impossible to obtain proper cathode polarization which enables the electrodeposition of titanium metal in a compact form.

Fluctuating current of the following types can be used to perform the above mentioned deposition of titanium metal:

(1) The currents passing through the cathode surface are made direct currents of a definite direction, and the density of said direct currents is varied in definite patterns.

.(2) The currents passing through the cathode surface are made direct currents of a definite direction, the density of said direct currents being constant, and said currents are passed intermittently.

(3) Alternating currents superimposed on direct currents will have the same effect as the current type described in the above (1). t

(4) In the fluctuating current types as set forth above are incorporated the periods in which a use is made of the currents having an opposite polarity to that of the electrolyzing currents.

Among the components of the fluctuating current systems described above, the most important and indispensible is the intermittent electrolyzing currents, the function of which is to deposit magnesium and titanium on the cathode. To produce proper deposition on the cathode surface of magnesium and titanium, the electrolyzing currents should have adequate voltages and current densities at the cathode. Detailed description of the fluctuating currents capable of achieving the desired effects is disclosed hereinafter, in connection Withthe description of electrolytic baths.

I, the present inventor, hypothesized that if, by the balance of the metal-depositing action of the electrolyzing currents and the metal-corroding action of the electrolytic bath, polarization can be created on the surface of the cathode, such polarization should act to suppress the growth of the deposit at the projections and, at the same time, should promote the growth at the depressions; thus, it should tend to produce a compact deposit. 1

On this working hypothesis experiments were conducted to determine the optimum conditions of the electrolytic operation, such as the composition of the'electrolytic baths, and the nature of the fluctuating current systems. As expected, conditions have been found under which electrodeposition of titanium metal in compact form can be obtained with good reproductibility.

I found that the necessary polarization can be maintained on the surface of the cathode by the alternating deposition and rehalogenization of magnesium: this action of magnesium is controlled by the fluctuating currents of electrolysis, as mentioned earlier.

Generally speaking, the electrolytic baths should, at the operating temperature, have such fluidity as, to ,permit adequate difiusion of the bath components and the bath should exert a stabilizing effect on the titanium bromides, thus preventing their thermal decomposition at the operating temperature.

Moreover, the corroding action of an unstable electrolytic bath on the deposited metals should not be too violent.

Too severe corroding action of the bath will not be properly balanced by the corrosion-preventing action of the cathode polarization. It has been found that electrolytic baths with an excessive corroding action tend to produce in the electrodeposited titanium metal a dendritic structure which is undesirable. On the other hand, the bath must have enough corroding action to dissolve, with adequate velocity, the deposited titanium thatis not effectively protected by the cathode polarization, namely, the projecting parts of the deposit.

From this point of view, the fused salt electrolytic baths consisting of metal chlorides were judged unsuitable, because it has been found that they possess undesirably 'severe corroding action on the electrodeposited titanium.

The fused salt baths consisting of the bromides of magnesium, barium and sodium have been found to be endowed with proper amount of corroding action on titanium metal. Moreover, such baths facilitate the formation and maintenance of useful cathode polarization.

Composition of the fused metal bromides baths of the type mentioned above, which has been found most effective in electrodepositing titanium metal in compact forms is given in the following:

(A) 7 Parts BaBT MgBI'z is favored in the range of 400-600 C. Hence, at the operating temperature in excess of 600 C. or below 400 C., the baths will not have adequate corroding action on titanium, the Reaction 1 being the mechanism of chemical corrosion. When titanium bromides are added to the fused bromides baths of the composition specified above, the resultant mixtures form stable molten systems at the desired operating temperature range of 400-600 C.

More specifically, the mixture A can be modified to contain, 6-13 parts of barium bromide, 10-20 parts of magnesium bromide and parts of sodium bromide in molar ratio. Changing the proportion of the mixture beyond the specified limits enhanced the tendency of the electrolytic bath to produce titanium metal which has dendritic structure and, hence, rough surface.

The three components-mixtures of the composition specified above all have freezing point of less than 600 C.

It has now been found that, as the molar ratio of magnesium bromide is increased in the composition of said bromides bath, the ratio of the amount of dissolved titanium dibromide to that of dissolved titanium tribromide is increased. This means that, when said bromides bath is used in electrodeposition of titanium, the relativeconcentration of titanium dibromide over titanium tribromide is higher within the polarizing film than in other parts of the electrolytic bath; within the cathode polarization the concentration of magnesium bromide is elevated, as will be discussed later. This phenomenon of relative scarcity of titanium tribromide within the cathode polarization will result in the relatively slower rate of corrosion of the deposited titanium, which takes place mainly according to the aforesaid Equation 1, within the polarizing film than without. Therefore, when the .elec- Parts BaBr 6 MgBr; 7 NaBr 15 (Expressed in molar ratio) It is apparent by compairing the compositions of the quasieutectic mixture (mixture B) and the mixture which is practically more useful (mixture A), that addition of magnesium bromide to the mixture A will result in higher freezing point of the system. Therefore, as the concentration of magnesium bromide is elevated at the cathode surface, a part of the accumulated magnesium bromide will form colloidal mass of solids near the points where the current desnsity is relatively high. Net effect of this phenomenon is a tendency to equalize current density across the surface of the cathode, thus helping the formation of compact titanium deposit.

Voltage of the electrolyzing currents employed in the electrolytic operation l.0 volt) is high enough, under the conditions of the operation, employing a soluble anode to electrodeposit magnesium metal.

When the voltage of the electrolyzing current is lowered below the level required for the deposition of magnesium, magnesium metal is expected to be redissolved faster than titanium, because of the higher basicity of magnesium.

That magnesium bromide is enriched in the cathode polarization is attested by the following observation.

Thus, the cathode was withdrawn from the electrolytic bath of the composition given in Example 4, after electrolysis had been continued for 30 minutes at 440 C. Analysis of the thin (about 0.4 mm. in thickness) film of solidified electrolytic bath, which coated the surface of the cathode, revealed that ratio of magnesium bromide and barium bromide was 2.1 times the ratio found in samples taken from other parts of the bath. However, the ratio of barium bromide and sodium bromide did not show such local variation.

The three components-bath of the composition (A), or its usable modifications as defined previously, can be added to the compound bath consisting of the halides of potassium and lithium, or other compound salt baths into which titanium trihalides can be dissolved, to improve the latters potency in producing titanium metal of desired compactness. Increase in the proportion of the barium-magnesium-sodium mixture in such electrolytic bath facilitates the deposition of titanium in compact forms.

On the other hand, the electrolytic baths consisting of the bromides of barium, magnesium and sodium can be improved, in terms of their ability to produce titanium metal of desired compactness, by the addition of another three components-mixture of the following composition:

mixture does not exceed 600 C. Addition of the cesiummagnesium-sodium mixture (mixture C) to less than percent of the total electrolytic bath results in maximum improvement obtainable: use of such electrolytic bath mixture gives excellent reproducibility to the production of compact titanium metal with specular surface.

This phenomenon will be perhaps based upon the fact that the fluidity of the barium-magnesium-sodium mixture (mixture A) or its usable modification is increased by the addition of the cesium-magnesium-sodium mixture (mixture C).

It has been found that up to five-sixths of cesium chloride in the mixture C can be replaced by a quasi-eutectic mixture of lithium chloride and potassium chloride, without deteriorating the usefulness of the mixture CL Corroding action on titanium of electrolytic baths consisting of the bromides of barium, magnesium and sodium (mixture A or its usable modification) can be suppressed by the addition of calcium chloride. Further, it is, of course, necessary to prevent the titanium halides from being oxidized, by passing an inert gas such as argon and helium over the surface of electrolyte containing the titanium halides.

The fluctuating currents employed in the present invention is described in detail in the following.

Naturally, various parameters of the fluctuating currents, such as the intermitting frequency, mode of fluctuation and the like, must be modified in relation with the bath compositions, temperature, and other electrolytic conditions, for instance such factors as the extent of diffusion, convection and agitation of the electrolyte. To effect deposition of magnesium at the cathode, voltage of the electrolyzing current must be higher than the decomposition voltage of magnesium bromide, which is approximately 2.2 volts at the operating temperature of 400 C.

However, this value is lowered by the bromination reaction of titanium or its bromides at the anode.

Thus, if an insoluble anode is used, 1.0 volt supplied to the electrodes is high enough for the deposition of magnesium; in case a soluble anode (titanium) is used, 0.45 volt is suflicient.

Since electrolyzing currents of the voltage higher than 1.0 volt are used in practice, in case soluble anode is used, the eflfective voltage is higher than the voltage for the decompositionof magnesium bromide, even when the resistance of electrolytic bath is taken into consideration. I have found that the difference between soluble anode and insoluble anode brings no different results on the electro-deposited products.

To effect the deposition of magnesium, current density of the electrolyzing current must be greater than 6 amperes/dmfi: at lower current densities deposition of magnesium metal is suppressed because the competing reaction which reduces titaniumgbromides dominates.

The minimum required voltage to obtain current density of greater than 6 a-mperes/dm. is 1.0 volt, at the operating temperature of electrolysis.

It is immaterial to the present invention' whether the electrolyzing current as described above is alternated with an electric current of lowered voltage, or of reversed polarity, or with a period where no current is passed through the cell, provided such current does not electrodeposit magnesium. Corroding action of the electrolytic bath on the deposited metals is greater when positive current of lower voltage, or the reverse current of higher voltage is employed. In the case where strong corroding action is desirable to effect surface polishing of the product, the reverse current is applied. The reverse current may be supplied from an outside source, as shown in Fig. 3, or may be supplied with the electromotive force generated in the electrolytic system itself, as shown in Fig. 2.

Frequency of the fluctuating current must be above 100 cycles per minute. In the case where frequency of the fluctuating current is less than cycles per minute, electrodeposition of titanium metal is discontinued after short period of time, like the case with non-fluctuating direct current is used, due to excessive concentration-polarization. If, under such condition, the polarization is forcibly removed by agitation of the electrolytic bath or by lengthening the period of depolarization, dendritic growth of titanium is effected. On the other hand, in the event that there is any tendency to create excessive polarization in use of the fluctuating current higher than 100 cycles per minute, the optimum polarization can be maintained by vibrating the cathode under adequate conditions relative to the frequency and amplitude.

The ratio of the duration of the electrolyzing current, capable of depositing magnesium and titanium, to the length of the period wherein said current is discontinued must be properly adjusted in order to obtain the desired effect. As such ratio is progressively decreased, the tendency to produce titanium with rough surface increases. This, however, can be counteracted by increasing the density of the electrolyzing current.

Figure la is an electric arrangement diagram wherein the circuit of the electromotive force between electrodes is not provided outside the cell during the time of breaking of electrolyzing currents.

An electric voltage is to be supplied to anode 2 and cathode 3 of an electrolytic cell 1 through potentiometer 4 from electric source E. Make and break device 5 is provided on the side where the electrolytic cell 1 is. In this diagram, A is an ammeter and V is a voltmeter.

Figure 2a is an electric arrangement diagram wherein the-circuit for passing currents due to the electromotive force between electrodes is provided during the time of the breaking of electric currents.

That is to say, Figure 2a shows an electric arrangement wherein make and break device 5 in Figure 1a is arranged on the side where the electric source E is and, when the electrodepositing currents are breaking, the action of the corroding currents of the reverse direction, running through part r of potentiometer 4 due to the electromotive force generated between electrodes 2 and 3, is used.

Figure 3a is an electric arrangement diagram of a case where electrodepositing currents and corroding currents of reverse polarity are supplied alternately. Electrodeposit-ing currents of a suitable value are supplied to anode 2 and cathode 3 in electrolytic cell 1 through potentiometer 6 from electric source E The duration of passage of electricity is controlled by make and break device 7. From electric source E corroding currents of reverse polarity are supplied to electrolytic electrodes 2 and 3 through potentiometer 8 and make and break device 9. Make and break devices 7 and 9 are so correlated as to open 9 when the make and break device 7 is closed and vice versa.

Now, barium bromide,magnesium bromide and sodium bromide mixture (hereinafter shortly referred to as B.M.N.) and cesium chloride, magnesium chloride and sodium chloride mixture (hereinafter shortly referred to as C.M.N.) as mentioned in the bath compositions of the following examples shall have the following compositions.

B.M.N.: Parts BaBr ..11-l2 MgBr l0 NaBr l5 Parts CsCl 3 MgCl 2 NaCl I 2 (Expressed in molar ratio) a 17-18 parts (in moles) of TiCl were added to 100 parts (in moles) of the above mixture to increase the baths corroding action.

However, in this case, CsCl can be substituted up to 6 (expressed in molar ratio) by the two components system about [60 LiCl+40 KCl].

Now, examples of experiments made by using electric arrangement shown in Fig. la are given in the following:

Example 1 (1) Electrolytic cell: Test tube made of special glass,

50 mm. in inside diameter and 250 mm. high.

(2) Bath compositions:

LiBr, 45.9 parts KBr, 30.6 parts SrBr 13.5 parts B.M.N., 10.0 parts (Expressed in molar ratio) parts (in moles) of TiBr were added to 100 parts (in moles) of the above mentioned bath.

Bath temperature: 440 C.

Electrodes:

Anode: Titanium plate, 1.5 mm. thick and 14 mm.

wide. Length of the immersed part 30 mm.

Cathode: Titanium plate, 0.5 mm. thick and 8.5 mm.

wide. Length of the immersed part 30 mm. (Surface area 5.4 cm. Distance between electrodes 25 mm.

Electrolyzing current:

Intermittent currents (direct currents).

Rectangular wave, cut connection Frequency of make and break, 4000 times/minute.

Currents 0.54 amp. (average).

Density of currents 10 amp/din. (average).

Cell voltage: 1.7 volts. j

(6) Duration of electrolysis: 1 hour 30 minutes.

(7) State of electrodeposition: A plate of titanium with fiat smooth surfaces was obtained.

Example 2 (l) Electrolytic cell: The same as in Example 1. (2) Bath compositions:

B.M.N., 98 parts C.M.N., 2 parts (Expressed in molar ratio) 10 parts (in moles) of TiBr were added to 100 parts (in moles) of the above mentioned bath. Bath temperature: 520 C. at the time of starting the electrolysis. 440 C. at the time of thick electrodeposition.

560 C. at the time of the end of the electrolysis.

Electrodes: I Anode: Titanium plate, 3.0 mm. thick and 14 mm.

Wide. Length of immersed portion 50 mm. Cathode: Titanium plate, 0.3 mm. thick and 10 mm.

wide. Length of immersed portion 10 mm. Distance between electrodes 20 mm. Electrolyzing currents: Intermittent currents, rectangular waves, cut /3 connection ,6. Frequency of make and break, 4000 times/minute. Currents 0.3 amp. (average). Density of currents: amp./dm. (average). Cell voltage: 2.6 volt. Duration of electrolysis: 4 hours. State of electrodeposition: A flat plate of titanium with specular surfaces was electrodeposited. An example made by using the electric arrangement shown in Figure 2a is given in the following:

Example 3 (Potentiometer r showed 10 ohms) (1) Electrolytic cell: The same as in Example 1. (2) Bath compositions: The same as in Example 1.

(3) Bath temperatures: 400 C.

(4) Electrodes:

Anode: Titanium plate, 1.5 mm. thick and 14 mm.

wide. Length of the immersed part 30 mm.

Cathode: Titanium plate, 0.5 mm. thick and 7 mm. wide. Length of the immersed part 2-3 mm. (Surface area 3.52 cm.'.) Distance between electrodes 25 mm.

Electrolyzing currents:

Intermittent currents (direct currents).

Rectangular wave, cut Vs, connection /s.

Frequency of make and break, 4000 times/ minute.

Currents 0.53 amp. (average).

Density of currents 15 amp./dm. (average).

Cell voltage: 2.4 volts.

Duration of electrolysis: 1 hour.

State of electrodeposition:

A flat plate of titanium consisting of column-like crystals was obtained.

An example using the electric arrangement shown in Fig. 3a is shown in the following:

Example 4 1) Electrolytic cell: Quartz crucible, 150 mm. in inside diameter and 300 mm. high. (2) Bath compositions:

B.M.N., 96 parts C.M.N., 4 parts CaCl 30 parts (Expressed in molar ratio).

10 parts (in moles) of TlBI'g were added to 100 parts (in moles) of the above mentioned bath.

Bath temperature:

At the time of starting electrolysis 520 C.; at the time of thick electrodeposition 440 C. (Duration 15 minutes.) At the time of finishing electrolysis 560 C. (Duration 20 minutes.)

Electrodes:

Anode: Titanium plate, 6 mm. thick, 50 mm. wide and length of the immersed part 50 mm.

Cathode: Titanium plate, 0.3 mm. thick, 50 mm. wide and length of the immersed part 50 mm. (Area 50.6 cm.

Electrolyzing currents:

Intermittent currents (direct currents), out 4, connection Frequency of make and break, 7000 times/minute.

Right direction currents (depositing) 5 amp. (average).

Density of currents (depositing) 10 amp./dm.

(average).

Reverse direction currents (corroding) 1.5 amp.

(average).

Density of currents (corroding) 3 amp./dm. (average).

Cell voltage (depositing): 1.9 volts.

(6) Duration of electrolysis: 20 hours.

(7) State of electrolysis: A thick fiat plate of'titanium with specular surfaces was obtained.

As is clear from the above examples, the electrodeposition of titanium in a compact form can be easily effected, although it has hitherto been considered 'difficult.

Further, though the electrolyzing currents and other currents were made to alternate with the same frequency in the above Examples 3 and 4, such alternation need not always to have the same frequency but may be adequately selected according to the operation, conditions and the like of the electrolysis.

Principle of the present invention may well be applicable to the electrodeposition of zirconium, hafnium and thorium which belong to the fourth group in the periodic table and thus have similar properties with titanium. I

. Further application of the present invention may be the electrodeposition of the alloys of titanium. Composition of the alloys may be controlled, to a certain extent, by the composition of electrolytic bath.

This application is a continuation-in-part of my application, Serial No. 425,600, filed April 26, 1954, now abandoned.

What I claim is:

1. A method of electrodepositing titanium which comprises passing through a fused salt bath containing at least a mixture composed, in molar ratio, of 6 to 13 parts of barium bromide, 10 to 20 parts of magnesium bromide and 15 parts of sodium bromide, admixed with titanium bromides, a fluctuating current having an intermittent current component having a frequency of at least one hundred per minute, a cell voltage higher than at least 1.0 volt and a cathode current density higher than at least 6 amp./dm. to deposit magnesium metal on the cathode, together with titanium metal.

2. A method of electrodeposition of titanium as defined in claim 1 which comprises passing through said fused salt bath intermittently interrupted electrolyzing currents having a cell voltage higher than at least 1.0 volt and a cathode current density higher than at least 6 amp./dm. to deposit on the cathode at least magnesium metal together with titanium metal.

3. A method of electrodeposition of titaninum as defined in claim 1 which comprises passing through said fused salt bath electrolyzing currents having a cell voltage higher than at least 1.0 volt and a cathode current density exceeding at least 6 amp./dm. to deposit on the cathode at least magnesium metal together with titanium metal for a predetermined time and reducing the cathode current density of said electrolyzing currents to less than 6 amp./dm. so that the aforesaid currents will not deposit magnesium.

4. A method of electrodeposition of titanium as defined in claim 2 which comprises bridging the terminals of the electrolytic electrodes through a variable resistance during the time of said interruption to pass currents in the direction opposite to that of said electrolyzing currents, due to the electromotive force being generated between the aforesaid terminals.

5. A method of electrodeposition of titanium as delined in claim 2 which comprises passing currents in the direction opposite to that of said electrolyzing currents between the terminals of the electrolytic electrodes during the time of said interruption to dissolve the projecting parts on the surface of the deposited metallic titanium and to make said deposited surface more smooth.

References Cited in the file of this patent UNITED STATES PATENTS 1,534,709 Holt Apr. 21, 1925 2,443,599 Chester June 28, 1948 2,515,192 Chester July 18, 1950 2,734,856 Schultz et a1 Feb. 14, 1956 2,741,588 Alpert et a1. Apr. 10, 1956 2,749,295 Svanstrom et al. June 5, 1956 OTHER REFERENCES Almand: Applied Electrochemistry, pp. 147-449.

Claims (1)

1. A METHOD OF ELECTRODEPOSITING TITANIUM WHICH COMPRISES PASSING THROUGH A FUSED SALT BATH CONTAINING AT LEAST A MIXTURE COMPOSED, IN MOLAR RATIO, OF 6 TO 13 PARTS OF BARIUM BROMIDE, 10 TO 20 PARTS OF MAGNESIUM BROMIDE AND 15 PARTS OF SODIUM BROMIDE, ADMIXED WITH TITANIUM BROMIDES, A FLUCTUATING CURRENT HAVING AN INTERMITTENT CURRENT COMPONENT HAVING A FREQUENCY OF AT LEAST

Images