US2745802A - Inorganic molten electrolyte for the electrolysis of titanium - Google Patents

Description

May 15, 1956 W. SCHMIDT INORGANIC MOLTEN ELECTROLYTE FOR THE ELECTROLYSIS OF TITANIUM H TTRNEH May 15, 1956 w, SCHMIDT 2,745,802

INORGANIC MOLTEN ELECTROLYTE FOR THE ELECTROLYSIS OF TITANIUM Filed Sept. 18, 1952 2 Sheets-Sheet 2 MELT GROUP I: Ti m4 Al POwDER A| :|3 -cATALYsT 7o v., Al cl3 2o Nu cl l lo v. K cl I REFLuxcONDENsER REAcTOR (|30 c) coNDENsER FOR REMOVAL OF T C|3+AI cl3 T. c|4 ExcEss AFTER REACTION (PLUS si c14- IMPURITY) T MIXER (leve) DECANTED LIQUID DECANTAT'QN (so A Al cl3, sEPARATOR o.o3/.Ti cl3, @00%) REST Na cl Kc|,PLus F cl3 IMPURITY) sEPARATEO MELT GROUP n:

sOLlD T; cl3 La cl-Kcl REGLAMATION PLus ADHERENT EuTEcTlc FOR RE-usE MELT. (40o c) MlxER OONDENSER REOLAMATTON oF Al cl3 AND SOME IN V EN TOR.

V WA HER SCHMIDT Ti ELECTROLYTE BY /vul-a ATTORN EY INORGANIC MQLTEN ELECTROLYTE FOR THE ELECTROLYSIS OF TITANIUIVI Walther Schmidt, Richmond, Va., assigner to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Application September 18, 1952, Serial No. 310,365

7 Claims. (Cl. 204-64) This invention relates to preparing an inorganic molten electrolyte for the electrolysis of titanium. Previous attempts have been made to electrolyze Ti from inorganic salt melts containing TiCla. The entectic of KCl and LiCl has been used for dissolving TiCls followed by electrolysis of the melt, using electrodes of tungsten and a temperature range of 500-550 C. Pure anhydrous TiCls is needed for such operations. This was prepared by reacting purified TiCl4 with hydrogen according to the reversible reaction:

TiClt-l-HTiCls -l-HCl This reaction proceeds in the desired direction, forming TiCla, at a temperature above 500 C. The products to the left side of the equation are reformed upon cooling. lt is, therefore, necessary to counteract this reformation by quenching, but the yield is never complete, and the process yields substantial amounts of all four substances in the final product, ln addition, TiClz is also formed, and its formation can be suppressed only with the use of still higher temperatures, e.g. 800 C. lt is, however, di'icult to work with the corrosive HCl and explosive H at high temperatures. This process of forming TiCls. is, clearly, costly with respect to energy consumption, yield, and maintenance of equipment.

lt is also known that anhydrous TiCl3 can be formed at much lower temperatures from TiCl4 by the reducing action of many metals. On example is:

This reaction proceeds in the desired direction, rather slowly at normal temperature, and quite speedily at about 100 C., the speed depending mostly on distribution, eg. by stirring. The reaction reverses at high temperatures, e.g. above 180 C.

Another example is the reaction:

3TiCl4+Al=3 TiCls+AlCla This reaction proceeds with Al-powder slowly at a temperature of about 130 C. The speed of the reaction is considerably increased at 180 C.-200 C. The conditions for reversing this reaction are not well established as to temperature and speed, but indications are that it reverses slowly at higher temperatures, probably above 220-250 C., especially if AlCla and TiCla are in contact within a salt melt.

A combination of these metals may be used for breaking down TiCLi, c g. Al-amalgam.

All of these reducing reactions proceed at low temperatures. They do not require much energy, are easily controlled as to reversibility and speed, and can be accomplished with low-cost equipment, as compared with the use of hydrogen as the reducing agent.

These reactions, however, do not produce pure TiCla, but a mixture of TiClz and the equivalent amount of chloride of the reducing metalA The literature also reports eorts to separate these chlorides, e.g. the attempt to use Al for the reduction and to remove AlC13 by dis- Sitatcs Patent F 2,745,802 Patented May 15, 1956 tillation. The aim was, however, always directed to the goal of recovering TiCl3 as a pure chemical.

This invention is described with reference to the accompanying drawings, in which,

Fig. l is a dow sheet using Hg as a reducing metal;

Fig. 2 is a ow sheet using Al as a reducing metal.

ln both Figs. 1 and 2 a carrier melt of the type of group i, consisting of AlCls plus a minor quantity of halides of alkali metals is employed for removing a major part of the HgClz (in Fig. 1) or AlCla (in Pig. 2) originating from the reaction of TiCl4 with Hg or Al. Further purification in both cases is accomplished by distillation after the major part of HgClz or AlCls has been selectively dissolved in that carrier, and undissolved TiCls has been separated.

Fig. l shows the reversal of the reaction, given on page 2, for purilication during a separate distillation step.

Fig. 2 shows the purification by distilling off vapors of AlCla and some TiClc, the latter being formed by reversed reaction at the higher temperature at which the TiCls is mixed to the final carrier-electrolyte.

In both cases the final electrolyte is one of group Ii, comprising an anhydrous inorganic melt of halides less noble than Ti, excluding AlCla, however, in amounts larger than a minor contamination.

This invention makes use of the known reactions with the purpose of developing a suitable electrolyte. lt was found that in many cases it would not be necessary to separate TiCl3 from the chloride of the reducing metal, but an electrolyte could be prepared by adding with mixing both the TiClg and the chloride of the reducing metal to a carrier electrolyte, the latter consisting of halides of metals less noble than Ti.

As indicated above, two groups of carriers are used in the process, the irst one serving as an intermediate melt for the purpose of selective dissolution and separation, the second one being the linal electrolyte. The first group contains a substantial, preferably a predominant amount of AlCla, e.g. the binary or ternary eutectics of AlCls with NaCl, KCl, and LiCl. Carriers of this group can be made to have a melting point as low as 70c C., e. g. the ternary eutectic of AlCl3, NaCl, KCl containing approximately AlCls, 10% KCl, 10% NaCl. AlCls and most of the added chlorides, e.g. all the alkali metal chlorides, form double salts when dissolved in each other. The second group does not contain AlCls except amounts which may represent unintentional contamination. Such electrolytes are, eg. the ternary eutectic of LiCl, RC1, NaCl, or the binary eutectic of LiCl and KCl or of NaCl and BeClz.

The first group of carriers has the advantage that it is molten at a temperature below the temperature at which the dissolved chlorides of the reducing metal and TiCla would reverse the reaction, forming that metal and TiCl4. The solubility of TiCla at this low temperature is rather low.

The use of carriers of the first group is limited to a range of temperatures, which are determined by the factors inuencing the reversing of the reaction between TiCl3 and the metal chloride. Since the formed TiCL; vaporizes away, the reverse reaction Would proceed as soon as unfavorable conditions of temperature and concentration were established.

Carriers of the second group have been used before. They have higher solubility for TiCla. The absence of AlCla allows a larger range of voltage in the cell. Carriers of the second group, however, can only be used when chlorides of reducing metals are removed, since they would, at the higher melting temperature, destroy the TiCla to reform TiCl4, which latter would vaporize out of the carrier.

The low melting point of a carrier of the first group allows it to be molten at a temperature at which TiCls is but slightly soluble. At a temperature of 100 C., using for example a carrier of 77% AlCls, 10% NaCl, KCl, and 3% LiCl, the solubility of TiCla is only about 0.03%, while the solubility of other metal chlorides, e. g. HgClz or FeCla, is much higher. Another example is a carrier-melt consisting of 75% AlCls, 15% NaCl, and 10% KCl, in which the estimated solubility at 180 C. is between .05 and .15% TiCls. The composition of the carrier, therefore, can be chosen within limits, in which most of the TiCls stays undissolved, while the chloride of the reducing metal is dissolved and can be separated by filtering, e. g. through ceramic filters. Other means of separation, like decantation or centrifuging, may be substituted for or be combined with filtration.

If Al-powdcr is used for breaking down TiCl4, often some particles remain unreacted or are covered with a crust of TiCls, thereby hindering complete reaction. The separated residue, therefore, may contain residual amounts of the carrier of the first group, unreacted metal, residual amounts of occluded chlorides of these metals, and TiCla.

TiCla from this residue is dissolved in a carrier, of the second group. At higher melting temperature of this type of carrier residual amounts of AlCls may vaporize. The minor part of the original amount of AlCla, being still present, is allowed to either vaporize or back-react with TiCla. Thus, a small part of TiCla is lost during purification. The TiCl4 formed is collected and recycled. AlCls thus remains only in minor amounts as unintentional contamination. Metallic residues and metals formed by reversible reaction are filtered off and the carrier is then ready for electrolysis (see Fig. 2).

This method offers the advantage of combining the preparation of an electrolyte with purification of TiC14. The purification of TiCl4 is dfcult. The literature offers many suggestions, most of them involving fractional distillation combined with chemical steps of purification. The fractional distillation is difficult because the partial pressure of other chlorides commonly present in TiCl4, is sufficiently high to cause trouble. Silicon tetrachloride, though boiling lower than TiCl4, contains always large amounts of TiCl4. upon removal. Repeated distillations are needed to eliminate SiCl4 and to recover a satisfactory yield of TiCl4. These difficulties are best illustrated by the fact that water-white TiCl4 of a purity suitable for Ti-production is available on the U. S. market only at a price which is more than 50% higher than for crude yellow TiCl4, which can be received without too much difficulty. The method described above offers simultaneously a method for purification of such crude TiCl4. The problem of the partial pressure of TiCl4 in relation to that of SiCl4 does not matter after the TiCl4 has been converted into TiCls. Other metallic chlorides dissolve readily in the carrier of the first group. Their solubility is much higher than that of TiCls. They are removed along with the chlorides of reducing metals. The carrier can be freed from them and adjusted by known means, like cementation, electrolysis, or vaporization of AlCls. Hydrates, it' present in crude TiCl4, form sludge and stay with the TiCla residue which is filtered from the carrier of the rst group. After this residue is dissolved in a carrier of the second group, this sludge is filtered off along with the metallic impurities.

Generally speaking, the low solubility of TiCla at low temperature in a molten carrier of the first group is used to leach out and separate the metallic chlorides of metals used to break down TiCl4 and soluble halides of impurities contained in the crude TiCl4.

The following is one example of carrying out the method described above. TiCl4 purified by common filtration and fractional distillation, or still containing other metallic chlorides and SiCl4 as impurities, is intimately mixed or contacted with mercury at about C. The mercury is used in excess, preferably in a column, the TiCl4 being introduced from underneath its surface, preferably from the bottom of a reaction vessel with provisions that it has to travel slowly enough and with sufficient contact to react as completely as possible. On top of the mercury a melt is kept floating consisting of the ternary eutectic of AlCla, KCl, NaCl, or a similar mixture, e. g. 77 AlCls, 10% KCl, 10% NaCl, and 3% LiCl. Unreacted TiCli is only slightly soluble in this mixture. Any excess will oat atop of it and can be removed by overfiow. Any vapors of TiCl4, SiCLi and AlCls are condensed. They may be purified by return to the crude TiCls and passing them through the preliminary purication.

The products of the reaction, namely TiCl3 and HgCl rise to the surface because of their buoyancy, where they contact the carrier. HgCl decomposes upon this contact and forms spontaneously Hg-i-HgClz. It seems that AlCls has a catalytical influence in promoting this decomposition. The mercury reformed by this reaction reunites with the mercury stock. The amount of carrier is large enoughl to dissolve all of the HgClz, while only a small part of the TiCla is dissolved. The remainder forms a suspension. By adding new or recycled carrier and moving the saturated carrier out of the vessel, circulation can be established, allowing the process to be carried out continuously.

This carrier is ready to be used as Such, preferably in a two-step electrolysis, first plating out the mercury, and thereafter the titanium. This method is most useful if Ti is plated out against a mercury-cathode. The electrolysis can be done in two different cells. The first cell is charged with carrier filtered off the original carrier. This offers the advantage that impurities from the crude TiCl4 may be deposited in the first cell and that different conditions of concentration, carriers, temperatures, cathodes, and electrical controls may be used in the cells.

The separated residue of TiCla is dissolved in a carrier of the second group, it being first separated from the above described melt by mechanical means such as decantation, filtration, centrifuging, or such means in combination. In this case it is advisable first to form a coarser particle size for easy filtering. This can be done by allowing the solid products in the reducing apparatus to dwell preventing them by mechanical means from rising to the surface of the mercury.

The mechanical separation is done at a temperature range of low solubility of TiCla, e. g. about 100 C. The amount of carrier of the first group is chosen large enough and the temperature should be low enough as to dissolve the chloride of the reducing metal, but not to dissolve substantial amounts of TiCla. The temperature should also not be so high as to endanger reformation of TiCli, e. g. not above C., in the case of the example.

The filtering may be done with ceramic filters, like alundum. Filtering aids, especially those based on diatomaceous silica, may be used to retain finer particles. The ltrate is readjusted by plating or depositing out the mercury and whatever impurities found in the filtrate. It is not necessary to deplete it, since it is recycled and reused. Thus it dissolves more HgClz in subsequent cycles, While the dissolved amount of TiCla remains unchanged. The

Amore noble impurities and the mercury do not require as high a decomposition voltage as TiCla. Thus, losses of Ti can be minimized. The mercury is cleaned from codeposited contaminating metals and is reused.

For use with a carrier of the second group, the filtered residue is heated in order to drive off AlCls and whatever amounts of TiCl4 or SiCl4. may still be present. The temperature is raised above 180 C. into the range of the reversed reaction between HgClz and TiCls in order to remove any occluded mercury-compound by reforming mercury and TiCli, which are driven oi and collected by condensation. The heating is preferably done in a separate step before dissolving the TiCla in a carrier of the second group, in order to avoid excessive formation of froth. A clean solution can be formed by a second iiltering through ceramic filters before electrolyzing. (See Fig. l.)

The following example shows the use of aluminum powder as reducing agent. Aluminum powder is reacted in the presence of some AlCla with TiCh at 130 C., using reflux conditions. Excess TiCli carries with it whatever amounts of SiClt that may be present in the raw material. The reaction can be speeded up by the use of higher temperatures, e. g., higher than the boiling point of TiCl4, and in a pressure vessel.

The mixture of AlCl3 and TiCl3 formed is dissolved in a carrier melt of the first group, containing only as much AlCls as is necessary to dissolve more of it. In this example a melt is used containing 70% AlCls, 20% NaCl, and KCl. The AlCla simultaneously formed with the TiCla is allowed to form a melt with the carrier, e. g. bringing the concentration from 70 to 80% AlCls. Impurities of chlorides of other metals may be dissolved in the carrier, and stay there, if they are less noble than Al. Nobler metals are deposited or cemented lout by some of the Al-powder during the foregoing reducing reaction and form sludge.

For preparing an electrolyte of the second group, the TiCls is separated by e. g. filtering from the rst melt at about 100 C.

The TiClg recovered by filtration is still contaminated with adherent carrier melt, unreacted Al-powder, and sludge. The contamination with adherent melt is Vcompatible with the electrolyte of the second group, in which the residue is dissolved. It is, however, advisable to drive oi AlCla from the residue by distillation before dissolving it in the iinal carrier. In this example the eutectic of KCl and LiCl is used as linal carrier, and a temperature above 400 C. is applied. The residue of unreacted A1- powder and sludge is filtered from the solution (see Fig. 2).

The preparation of an electrolyte for Ti, containing TiCla dissolved in an inorganic salt melt, as known to the art, involved the costly steps of purifying TiCl4, produc ing and purifying TiCls therefrom, and nally dissolving the pure T iClg chemical in a salt melt. This invention con bines the action of reducing metals with the action and selective properties of suitable salt melts. The different electrolytical potentials in such melts allow selective electrolysis. The different solubility at a given temperature allows selective separation of TiCla from different metallic compounds dissolved in such melts. These properties are not only used to reduce TiCl4 but also to purify it, while preparing an electrolyte containing TiCla, thereby saving costly steps otherwise required. The invention, therefore, shows a method of producing inorganic electrolytes suitable for the electrolytical recovery of Ti with considerable economical advantage.

Though the carriers described above are based on TiCla as the Ti-compound to be electrolyzed, some amounts of TiClz or TiCl.;t may be simultaneously present. When, e. g. Pil-powder is used as reducing metal, there is always some TiCl2 formed along with TiCls. There may also be a small amount of TiCl4 dissolved in the carrier. The scope of this invention is, therefore, not limited to TiCla alone, though this compound is the main constituent, desired by the invention. The rl`i-chlorides of lower or higher valency may, however, be left in the carrier without causing diiiiculties.

Having described my invention, what I claim and desire to secure by Letters Patent is as follows:

l. In a method of preparing an inorganic molten electrolyte for the purpose of electrolytically producing Ti, said electrolyte consisting substantially of a plurality of halides selected from the group of chlorides and uorides of metals less noble than Ti and containing TiCls being obtained by partial reduction of TiCl4 by reaction with metallic Al, said T iCls being therefore mixed with AlCls,

6., the improvement which comprises mixing said TiCls and AlCl3 with a melt consisting essentially of AlCla and at least one alkali chloride selected from the group consisting of NaCl, KCl, and LiCl, forming a melt in which the' quantity by weight of AlCla is larger than the quantity of said alkali chloride, applying a quantity of that melt suficiently large and at a temperature suiiciently low so as to dissolve said AlCla yet not to dissolve substantial amounts of TiCls, said temperature being also sufliciently low not to cause reformation or TiClr, separating the undissolved TiCls from the melt, thereafter forming said electrolyte by mixing the TiCls with said plurality of molten halides selected from the group of chlorides and iluorides of metals less noble than Ti, the amount of AlCla present in said electrolyte being limited to a minor amount such as being introduced as contamination along with the TiCla.

2. In a method of preparing an inorganic molten electrolyte for the purpose of electrolytically producing Ti, said electrolyte consisting of a plurality of halides selected from the group of chlorides and fluorides of metals less noble than Ti and containing TiCl3 being obtained by partial reduction of TiCl4 by reaction with a metal selected from the group consisting of Al, and Hg, said TiCl3 being therefore mixed with the corresponding chloride of the reducing metal, the improvement which cornprises mixing said TiCl3 and chloride of the reducing metal with a melt consisting essentially of AlCla and at least one alkali chloride selected from the group consisting of NaCl, KCl, and LiCl, forming a melt in which the quantity by weight of AlCl3 is larger than the quantity of said alkali chloride, applying a quantity of that melt sufficiently large and at a temperature sufliciently low so as to dissolve said chloride of reducing metal, yet not to dissolve substantial amounts of TiCla, said temperature being also sudciently low not to cause reformation of TiCl4, separating the undissolved TiCla from the melt, thereafter forming said electrolyte by mixing the TiCls with said plurality of molten halides selected from the group of chlorides and iluorides of metals less noble than Ti, the amount of AlCl3 present in said electrolyte being limited to a minor amount such as being introduced as contamination along with the TiCls.

3. In a method of preparing an inorganic molten electrolyte for the purpose of electrolytically producing Ti, said electrolyte comprising substantially the eutectic mixture of LiCl and KCl and containing TiCla, being obtained by partial reduction of TiClr by reaction with metallic Al, said TiCls therefore being originally contaminated with AlCl3 in the molar relation of three moles TiCla and one mole AlCls, the improvement which comprises substantially reducing the amount of said AlCls contamination by mixing said TiCl3 and AlCla with a melt consisting essentially of AlCls and at least one alkali chloride selected from the group consisting of NaCl, KCl, LiCl, forming a melt in which the quantity by weight of AlCla is larger than the quantity of said alkali-chloride, applying a quantity of said melt sufficiently large and at a temperature sutliciently low so as to dissolve all of said AlCl3 yet not to dissolve substantial amounts of TiClz, the temperature being also suiiiciently low not to cause reformation of TiCl4, separating the undissolved TiCls from the melt, and thereafter forming said electrolyte by mixing the separated TiCls with said molten eutectic mixture of LiCl and KCl.

4. A method for separating TiCl3 from the chloride of a reducing metal for the purpose of preparing said TiCls suitable to be fed into a molten inorganic electrolyte, said reducing metal being selected from the group comprising Al and Hg, said TiCl3 and chloride being simultaneously formed by reducing TiClr, which comprises dissolving fhe chloride of the reducing metal in a melt consisting essentially of a mixture of AlCls and at least one alkali-chloride selected from the group consisting of NaCl, KCl and LiCl, thereby forming a melt in which the quantity by weight of AlClg is larger than the quantity of said alkali-chloride, employing a quantity of said melt sufciently large and at a temperature suciently low, so as to dissolve all of said chloride of reducing metals, yet not to dissolve substantial amounts of TiCla, and separating the undissolved T iCla from the melt.

5. The method of separating TiCl3 from at least the major part of AlCls, said TiCl3 being obtained by reducing TiClt with aluminum, simultaneously forming a mixture having the proportion of 3 moles TiCls to 1 AlCls for the purpose of preparing T iCla suitable to be fed into a molten inorganic electrolyte, which comprises dissolving the AlCl3 by incorporating it into a melt consisting es'- sentially of a mixture of AlCla and at least one alkalichloride selected from the group consisting of NaCl, KCl and LiCl, thereby forming a melt' in which the quantity by weight of AlCla is larger than the quantity of said alkali-chloride, employing a quantity of said melt suiciently large and at a temperature suiciently low so as to dissolve all of the AlClx, yet not to dissolve subf stantial amounts of TiCla, and separating the undissolved TiCls from the melt.

6. In a method of preparing an inorganic molten electrolyte for the purpose of electrolytically producing titanium, said electrolyte consisting of a plurality of halides selected from the group of chlorides and fluorides of metals less noble than titanium and containing TiCla being obtained by partial reduction of TiCl4 by reaction with mercury, said TiCla being therefore mixed withV the corresponding chloride of the reducing metal, the unprovement which comprises reacting mercury with TiCl-r, forming TiCla and HgCl, while keeping floating on the top of the mercury a mixed melt consisting essentially of a major proportion by weight of AlCls, and a minor proportion by weight of at least one alkali-chloride selected from the group consisting ofNaCl, KCl, and LiCl, said melt having a temperature above its melting pointand below 180 C., allowing the products of the reaction to rise and to be mixed into said melt, the HgCl thereupon forming Hg and HgClz, employing a quantityof said melt sufficiently large so as to dissolve all of the HgClz yet not large enough to dissolve substantial amounts of TiCla, said temperature being also sufficiently low'not to cause reformation of TiCl4, separating the undissolved TiCla from the melt,v and thereafter forming said electrolyte by mixing the TiC13 with said plurality of molten halides selected from the group of chlorides and uorides of metals less noble than Ti, the amount of AlCla present in said electrolyte being limited to a minor amount such as being introduced as contamination along with the TiCls.

V7. In the methodof claim 6 the additional step which comprises purifying the TiClg after its separation from themelt in which the HgClZ has been dissolved, by heating it toz a temperature above 180 C., eiecting Vthe reversed reaction between TiCl3 and any occluded HgClz, vaporizing the reformed 'Hg and TiCli, and thereafter mixing the puried residue consisting mainly of TiCls with said plurality of molten halides.`

Claims (1)

1. 4. A METHOD FOR SEPARATING TICL3 FROM THE CHLORIDE OF A REDUCING METAL FOR THE PURPOSE OF PREPARING SAID TICL2 SUITABLE TO BE FED INTO A MOLTEN INORGANIC ELECTROLYTE, SAID REDUCING METAL BEING SELECTED FROM THE GROUP COMPRISING AL AND HG, SAID TICL3 AND CHLORIDE BEING SIMULTANEOUSLY FORMED BY REDUCING TICL4, WHICH COMPRISES DISSOLVING THE CHLORIDE OF THE REDUCING METAL IN A MELT CONSISTING ESSENTIALLY OF A MIXTURE OF ALCL3 AND AT LEAST ONE ALKALI-CHLORIDE SELECTED FROM THE GROUP CONSISTING OF NACL, KCL AND LICL, THEREBY FORMING A MELT IN WHICH THE QUANTITY BY WEIGHT OF ALCL3 IS LARGER THAN THE QUANTITY OF SAID ALKALI-CHLORIDE, EMPLOYING A QUANTITY OF SAID MELT SUFFICIENTLY LARGE AND AT A TEMPERATURE SUFFICIENTLY LOW, SO AS TO DISSOLVE ALL OF SAID CHLORIDE OF REDUCING METALS, YET NOT TO DISSOLVE SUBSTANTIAL AMOUNTS OF TICL3, AND SEPARATING THE UNDISSOLVED TICL3 FROM THE MELT.

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