US2880150A - Electrolytic method and apparatus - Google Patents

Description

March 31, 1959 Filed 0G12. 2.2, 1956 ELECTRC/)LYTIC METHOD AND APPARATUS F. A. HOWARD 2 sheets-sheet 1 v March 31, 1959 F. A. HOWARD 2588.0,150 ELECTROLYTIC METHOD AND APPARATUS Filed oct. 22, 195e v *l I 2 sheetssheet 2 Frank A. Howard BY X12/@AQ Vw/K United States Patent O 2,880,150 ELECTROLYTIC METHOD AND APPARATUS Frank A. Howard, New York, N.

Application October 22, 1956, Serial No. 617,339

4 Claims. (Cl. 204-64) This invention relates to the production of rare and refractory metals of groups IV-A, V-A, and VI-A of the periodic table. More particularly, it relates to a fused salt electrolysis wherein such metals as titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten and uranium may be recovered, and to the apparatus for carrying out such an electrolysis.

The electrolytic recovery of these rare and refractory metals by fused salt electrolysis has been described in a number of patents issued to Driggs and his coworkers during the 1930s, namely United States Patents 1,815,054; 1,821,176;`1,835,025; 1,861,625; 1,874,090 and others. The processes therein described may be summarized as fused salt electrolyses involving a molten bath composed of (1) a carrier or diluent salt comprising at least one -halide from the group of alkali metal halides and alkaline earth metal halides, and (2) a source of the desired refractory metals, such as a simple or complex halide or an oxide of the metal to be recovered.

In these and other more recently developed processes in which an inert atmosphere has been provided to improve the quality of the metal deposited, the desired metal is obtained in the form of a cathode deposit consisting of individual particles of the metal itself embedded in a mass of solidified salt from the molten bath.

Prior to my invention it has been the practice to separate the cathode deposit from the cathode on which it was deposited by mechanical means, such as stripping or scraping, and to recover the metal content of the metal-salt mass by dissolving out the salt by aqueous techniques or Vvolatilizing it by vacuum distillation. The freshly deposited metals and many of the compounds present in the salt mass may react with water or other leaching agents and with the metal to form either the oxide of the metal or hydrates or residues which are diflicult to separate from the desired metal value unless special precautions are taken. Vacuum distillation requires expensive apparatusand generally is effective only in removing a portion of the associated salt content. These and other prior art processing techniques which initially transported and later treated a solid mass of salt and metal also sufrered from contamination unless extraordinary precautions were taken to prevent contact of the metal with any oxidizing or otherwise active atmosphere during the handling ofthe deposit. Furthermore, the, salt content of the cathode deposit must be replaced in order to maintain uniform conditions inthe fused electrolytic bath and the values in the salt must be recovered as a matter of economics.

I have now devised a new system wherein the former methods of recovering the metal values in a cathode deposit are replaced by a means which effectively disintegrates the cathode deposit and separates the metal particles. Furthermore, in accordance with my invention most of the salt content of the cathode deposit and the values therein are separated from the metal portion of the deposit in a state in which the salt and the values 2,880,150 Patented Mar. 31, 1959 ice therein are readily returned to the electrolytic cell for reuse on an immediate basis.

One object of my invention is to provide for the immediate resolution of the salt content dragged out with 5 the cathode deposit when said deposit is withdrawn for recovery of its metal values.

Another object of my invention is to provide a relatively simple means in the form of a circulating stream of molten salt as al means for separating the metal values in the cathode deposit and for conveying the metal particles freed of the major portion of the salts forming a matrix in which they were embedded and transporting `the metal particles to a single separation point where the particles recovered from one or more cells may be removed from the system in a single processing unit.

These and other objects of my invention will be better understood by reference to the accompanying drawings in which:

Figure 1 shows in schematic fashion a plant for obtaining the several rare and refractory metals in accordance with the practice about to be described; and

Figure 2 is a diagrammatic view of a single electrolytic cell taken in section.

As shown in the plant of Figure 1, a plurality of cells are provided, arranged in one or more lines. Any convenient electrolytic cell may be used in the installation. The cells shown in Mellgren Patent 2,748,073 and that shown in Wainer Patent 2,707,170 are suitable if provided with an appropriate means for recovering the cathode deposit. Another suitable cell is shown in Figure 2. l The cell in Figure 2 comprises a vessel 10 provided with a conduit 12 to admit electrolyte into the vessel near'the bottom of the vessel 10 and with an exit tube 14 through which electrolyte may be continuously or intermittently removed from the upper portion of the vessel. The vessel is provided with a cover 18 carrying ports (not shown) for admitting and exhausting gases to and from the cell to maintain an inert atmosphere above the fused melt. The vessel 10 consists of a lower portion provided with an anodic lining 24, preferably of graphite, and an upper portion 11 which serves as the cathode removal chamber. One or more cathodes 26 are suspended through the cover 18. The cover is electrically insulated from container 10 by a gasket 15. A rack 20 and pinion 22, or other suitable means, effects raising and lowering of the cathode rod` to permit stripping of the cathode deposit 42 from the cathode rod 26. A gate valve 30 is adapted to be closed when the cathode rod 26 has been raised sutilciently to clear the valve. Severing means 44 is shown for mechanically detaching the cathode deposit. vAlternatively, a jet of molten salt admitted through a port 34 may be used to -detach the cathode deposit from the cathode rod 26. An outlet chute 38 is shown, through which the cathode deposit is passed to a hopper 41, through which passes a stream of molten salt which leads to a central processing station where the vsolid metal particles and the salt are separated by any suitable means. The conduit 46 serves as the connection between hopper 41 and the central station.

Means to supply heat and power to the cell have been omitted because such means are known-in the art.

As shown diagrammatically in Figure l, the electrolysis cells are provided with heated conduits 12 for admitting additional molten electrolyte and with cathode removal chambers 11 through which the cathode deposit may be withdrawn. The cathode deposit and the molten salt by which it is transported from the electrolysis cell pass from chamber 11, through chute 38 and conduit 46 and thence to a recovery station where the metal and the molten salt are separated by anyY suitable means 17, such as a rotary filter, or by a centrifuge, or by a gravity settling chamber. The metal particles so recovered may assenso be further processed by consolidation into ingots after the removal of any lilm or other small residue of salt attached thereto. Removal of any such small residue of adherent salt may be made by washing with a small amount of water, or by vacuum distillation or other suitable procedure.

The molten salt is pumped through line Ztl by pump 18 to a reaction vessel 22. The reactor is fo med of any suitable material of construction, such as graphite or ceramic, metallic or other refractory material. In the reactor 22 the molten salt is brought into contact with a suitable solid material added through a suitably valved solids feeding device Z4. Sutable solids for the reaction are those from which the refractory metal may be extracted by chemical reaction, as disclosed in French Patent 1,116,5l8, granted February 6, 1956, which acknowledges a corresponding application led in the United States on December 14, 1953. As disclosed therein, suitable metal-supplying solid materials include carbide of the metal or the metal itself in impure form or as alloy or commercial scrap metal, such as turnings or macerated plate scrap. A pump 26 and a filter 28, or other separatory means located in line 30 joining the reactor with the electrolytic cells, separate any unreacted solid material or any solid reaction products from the molten product being returned to the cells for further electrolysis. As shown, only a portion of the reconditioned salt is returned to the cells through lines 12. Part of the remainder is pumped through a heated conduit 32 to a cathode disintegrating hopper such as the hopper 41 of Figure 2 into which the cathode deposit is dumped when a suicient amount of metal has been deposited. The heated molten salt streams may be brought into contact with the cathode deposit in any convenient way to e'ect the distintegration and separation of the cathode deposit to recover the metal therein. In one modification shown in Figure 2, molten salt from line 32 may how through jets 34 to impinge on the cathode deposit and ush it off. The solid particles and salt stream leave the cell by chute 38. In another embodiment, the deposit is scraped and dumped into the cathode removal hopper dl as in Mellgren, where it is attacked by the hot salt streams flowing through hopper 41 and line do. The deposit and salt are withdrawn from hopper 41 by way of line 46. Either molten salt or a combined mechanical and uid means may thus cooperate to disengage the cathode deposit.

Still other methods are available for converting the cathode deposit from the solid mass of salt and metal particles into a molten salt stream carrying the solid metal particles in suspension. rIhus the cathode deposits from each cell can be detached from the cathode rod by heating the cathode to a temperature sutiieient to soften the salt of the salt-metal mass to permit the deposit to drop off the cathode and into the molten salt stream owing through the hopper 4l. Since the molten salt in lines 32 and 44 is already in equilibrium with the metal to be recovered by virtue of the reaction in reactor 22, the fluid in lines 32 and 44 does not react with or dissolve the metal in the cathode deposit. It mereiy melts the salt metal-mass and delivers the metal to separator 17. By suitably selecting the amount of salt introduced into chamber 11 through line 32, any tendency of the salt in the cathode deposit to react with the metal, upon remelting of the salt, may be minimized. Under the procedure described the circulating stream of iiuid electrolyte serves both to melt the cathode deposit at the individual cells and to transport it to a single separator.

In still another embodiment the cathode deposits are i stripped mechanically and mechanically transported to a common delivery point, where they are disintegrated by action of a heated molten salt. When so processed, the disintegrated deposit and molten salt pass on to the separator 17 as in the previous embodiments.

If the amount of electrolyte introduced into cell 10 exceeds that withdrawn with the cathode deposits, the excess can be removed through lines 14 connected to a header 40 which delivers the electrolyte to the reactor 22. Header 40 is provided in order that spent electrolyte withdrawn through lines 14 may be passed directly to the reactor 22 for regeneration, without intermingling with cathode deposited metal flowing through conduit 46.

In this way electrolyte in which the composition has been permitted to change to concentrations which would react with the freshly deposited metal do not contact such metal. Thus an individual cell about to be taken out of production may be electrolyzed to the point of exhaustion of its titanium content and the electrolyte values may then be salvaged through lines 14 and 40 before the cell is shut down.

Provision is made for the purge of a predetermined part of the content of the salt system, in order to avoid a build-up of contaminants therein and to prevent any unnecessary increase in the volume of salts being handled. Line 43 serves as the purge or bleeder for the unwanted salts.

A connection 44 has been provided between the fresh electrolyte supply line 30 and the conduit 46 by which the cathode deposits are conveyed to the separation station 17 in order that the circulation rate of electrolyte through conduit 40 may be subject to adjustment.

While many suggested electrolytic installations shown in patents have involved but a single cell, in large scale production it would be more advantageous economically to arrange a plurality of individual cells in line and to provide a common power supply, together with a common means for removing and for replenishing the individual materials furnished to the cell in order to convert an otherwise batch process into a continuous operation. Accordingly I have shown such a system, although it will be understood that my invention is also applicable to operation of the system with but a single cell. Similarly, it is possible to arrange the individual cells so that they have a common boundary as in Patent 2,748,073.

In order to more readily understand this invention it will be explained with reference to the production of titanium, although it will be understood by those skilled in the art that what is said is equally applicable to other refractory metals as indicated above.

In operation, a charge for the cell is made up by melting a suitable amount of diluent salt consisting of at least one alkali metal halide or alkaline earth metal halide. As described in Patent 2,731,402, when producing titanium, I prefer sodium chloride for reasons of economy and because this salt is readily obtainable in the desired purity. Since the bath salt must ultimately be melted to separate the cathode deposited metal from the salt mass concurrently solidified on the cathode during the electrolysis, I prefer to avoid the use of salt systems which are composed entirely of fluorides, and because of their inherently lower stability, I prefer to avoid the use of bromides and iodides. Accordingly I prefer alkali or alkaline earth chlorides, either singly or in admixture and particularly sodium chloride and/or potassiim chloride. The second constituent of the molten bath is a complex alkali metal-titanium double iluoride, preferably KZTiFG or NaZTiFG. The bath, prior to commencement of the electrolysis may be purified as described in Patent 2,731,402, but it is to be understood that such treatment is entirely optional in my system.

Once a suitable bath has been prepared, e.g. a bath consisting of 84% NaCl and 16% K2TiF6 (by weight), electrolysis is begun and fresh molten salt of appropriate composition is admitted through line 12. Initially a lower valent titanium compound may be prepared by reacting KZTiFe with scrap titanium or TiC as described in Patent 2,718,464, and the resulting compound may then be blended with sodium chloride in the desired proportions to produce a suitable feed for lines 32 and 12.

The blended feed is admitted to the cells 10 through inlet lines 12, in turn,rso that the operation of the cells will be staggered in point of time. The electrolysis is preferably carried out at current densities up to 400 amperes per square decimeter and with the bath temperature of between about 800 C. and 900 C. under an argon atmosphere. When the cell is properly operated no free halogen is evolved at the anode. Electrolye is continuously withdrawn through line 14 and returned, through header 40 to reactor 22, while line 46 is conveying spent electrolyte, fresh electrolyte and disintegrated cathode deposit to separator 17. After removal of any solids, the molten spent electrolyte is pumped by pump 18 to reactor 22 where it contacts either metallic titanium or titanium carbide and is regenerated for electrodeposition of titanium.

It will be seen that I have provided a system in which the cathode deposit is efficiently separated and little or no loss of electrolyte incurred when the metal content of the deposit is separately recovered. Furthermore, the cathode deposit is not subjected to exposure to contaminating inuences at any time prior to its separation and collection at a central station. e

Having now described my invention in accordance with the patent statutes, I claim:

1. In an electrolytic cell apparatus for obtaining a refractory metal by a fused salt electrolytic process, the improvements which comprise: a stripping chamber defined by an extension of the uppermost portion of the cell; means adapted to isolate said chamber from the remainder of the electrolytic cell; means to withdraw a cathode bearing a solid deposit of refractory metal particles and solidified salt electrolyte out of physical contact with the fused salt electrolyte and into said stripping chamber; means to admit at least one moving stream of molten salt into said stripping chamber and into phyical contact with said solid deposit, whereby the solidied salt content of said deposit is melted while the metal in said deposit remains in the form of discrete particles in the resulting molten salt medium; a conduit passing from said stripping chamber and adapted to contain a liowing stream of molten salt and to convey said mixture of molten salt and solid metal particles from said stripping chamber to a separating station; and means operatively connected to said conduit for separating the solid metal particles from the accompanying molten salt.

2. The apparatus of claim 1 in which the means for conveying is adapted to convey the cathode deposited metal particles and salt from more than one cathode to a single separating station.

3. Apparatus for obtaining a refractory metal by a fused salt electrolytic process comprising: a plurality of electrolytic cells each adapted to contain a fused salt melt and each provided with anode means, at least one cathode and means for passing an electrolyzing current in each cell between said anode means and at least one cathode in each cell whereby a solid deposit composedof refractory metal and solidified salt constituents of each salt melt is formed on each cathode; means for withdrawing said cathode and deposit from the melt; means for stripping said solid deposit of salt and metal from each withdrawn cathode; conduit means adapted to convey a molten salt stream connecting the individual cells; means for conveying said solidified deposit into said conduit means and into said salt stream after the deposit has been detached from its cathode, and wherein the solid deposit is transformed into solid metal particles in a melt of salt; other conduit means adapted to convey the disintegrated cathode deposit and molten salt stream to a processing station; means for separating the solid metal particles from the remainder of the melt and for recovering the separated metal particles substantially free from salt; and means for returning at least a portion of the salt to each cell in order that the process may be repeated.

4. The method of obtaining a refractory metal by a fused salt electrolysis comprising: passing an electrolyzing current between an anode and at least one cathode in electrical contact with a fused salt electrolyte contained in an electrolytic cell; withdrawing a cathode and the metal-salt deposit thereon from said fused salt electrolyte; separating said deposit from said cathode by applying a heated molten salt stream to said withdrawn deposit and thereby stripping and disintegrating said deposit and forming a salt melt in which the desired metal is present as discrete solid particles; separating said particles from said melt and recovering said refractory metal as said metal particles; and returning at least a portion of the molten salt to the electrolytic cell for a continuation of the process.

References Cited in the le of this patent UNITED STATES PATENTS 2,618,549 Glasser Nov. 18, 1952 2,748,073 Mellgren May 29, 1956 2,760,858 Findlay Aug. 28, 1956

Claims (2)

1. IN AN ELECTROLYTIC CELL APPARATUS FOR OBTAINING A REFRACTORY METAL BY A FUSED SALT ELECTROLYTIC PROCESS, THE IMPROVEMENTS WHICH COMPRISE: A STTIPPING CHAMBER DEFINED BY AN EXTENSION OF THE UPPERMOST PORTION OF THE CELL; MEANS ADAPTED TO ISOLATE SAID CHAMBER FROM THE REMAINDER OF THE ELECTROLYTIC CELL; MEANS TO WITHDRAW A CATHODE BEARING A SOLID DEPOSIT OF REFRACTORY METAL PARTICLES AND SOLIDIFIED SALT ELECTROLYTIC OUT OF PHYSICAL CONTACT WITH THE FUSED SALT ELECTROLYTE AND INTO SAID STRIPPING CHAMBER; MEANS TO ADMIT AT LEAST ONE MOVING STREAM OF MOLTEN SALT INTO SAID STRIPPING CHAMBER AND INTO PHYSICAL CONTACT WITH SAID SOLID DEPOSIT, WHEREBY THE SOLIDIFIED SALT CONTENT OF SAID DEPOSIT IS MELTED WHILE THE METAL IN SAID DEPOSIT REMAINS IN THE FORM OF DISCRETE PARTICLES IN THE RESULTING MOLTEN SALT MEDIUM; A CONDUIT PASSING FROM SAID STRIPPING CHAMBER AND ADAPTED TO CONTAIN A FLOWING STREAM OF MOLTEN SALT AND TO CONVEY SAID MIXTURE OF MOLTEN SALT AND SOLID METAL PARTICLES FROM SAID STRIPPING CHAMBER TO A SEPARATING STATION; AND MEANS OPERATIVELY CONNECTED TO SAID CONDUIT FOR SEPARATING THE SOLID METAL PARTICLES FROM THE ACCOMPANYING MOLTEN SALT.

4. THE METHOD OF OBTAINING A REFRACTORY METAL BY A FUSED SALT ELECTROLYSIS COMPRISING: PASSING AN ELECTROLYZING CURRENT BETWEEN AN ANODE AND AT LEAST ONE CATHODE IN ELECTRICAL CONTACT WITH A FUSED SALT ELECTROLYTE CONTAINED IN AN ELECTROLYTIC CELL; WITHDRAWING A CATHODE AND THE METAL-SALT DEPOSIT THEREON FROM SAID FUSED SALT ELECTROLYTE; SEPARATING SAID DEPOSIT FROM SAID CATHODE BY APPLYING A HEATED MOLTEN SALT STREAM TO SAID WITHDRAWN DEPOSIT AND THEREBY STRIPPING AND DISINTEGRATING SAID DEPOSIT AND FORMING A SALT MELT IN WHICH THE DESIRED METAL IS PRESENT AS DISCRETE SOLID PARTICLES; SEPARATING SAID PARTICLES FROM SAID MELT AND RCOVERING SAID REFRACTORY METAL AS SAID METAL PARTICLES; AND RETURNING AT LEAST A PORTION OF THE MOLTEN SALT TO THE ELECTROLYTIC CELL FOR A CONTINUATION OF THE PROCESS.

Images