NO813250L - PROCEDURE FOR DETERMINING THE SUITABILITY OF A METALLIC DIAPHRAGMA FOR USE IN AN ELECTROLYSIS CELL - Google Patents

Description

This invention relates to the production of polyvalent metals and particularly relates to a metal diaphragm and an apparatus and a method in which a metal diaphragm is used in the electrolytic production of a polyvalent metal from a salt thereof. The invention also includes an input cathode for use in the electrolytic production of such polyvalents. metals.

Polyvalent metals, such as titanium, have previously been prepared from compounds thereof, for example titanium tetrachloride,

by electrolysis, as described in US patent no. 2,789,943,

No. 2 943 032 and No. 3 082 159. Usually the titanium tetrachloride is introduced into a molten bath of alkali metal or alkaline earth metal salt through suitable means and electrolytically split so that the cathode is coated with metallic titanium and chlorine is developed at the anode. Various means have been used to separate the anode from the cathode in the titanium-containing electrolysis cells.

A physical barrier, such as a diaphragm, is required between the anode and cathode chambers to prevent titanium ions from flowing from the cathode chamber into the anode chamber. If a significant ion flow takes place, titanium ions will be oxidized to titanium tetrachloride and the cell's performance will be reduced. The diaphragm must allow the passage of chloride ions and molten salt bath between the anode compartment and the cathode compartment.

The diaphragm in US patent no. 2,789,943 consisted of a perforated, electrically conductive metal structure which in use could act alternately as cathode and anode. The diaphragm was used as a cathode for depositing metallic titanium in the diaphragm's pores and reducing the diaphragm's porosity. The electrical polarity was reversed, so that the diaphragm became the anode and titanium was removed from the diaphragm, when this had become too impermeable and reduced the efficiency of the electrolysis cell. Such a diaphragm with variable porosity can be used; however, it would be better to have a diaphragm that did not require constant monitoring and frequent metal deposition and metal removal...

The previously known electrolysis cells are operable; however, the barrier or diaphragm between the anode compartment and the cathode compartment has usually not had as good strength properties as required for electrolytic production equipment, or continuous and precise regulation has been necessary

of the porosity during the cell's operation. An improved electrolytic cell and method of operating an electrolytic cell for the extraction of metals during use of a diaphragm with satisfactory physical properties and constant porosity, which does not need to be regulated during operation, is desired.

Metals, such as titanium, have previously been produced

.electrolytically from, for example, titanium tetrachloride in molten salt baths, for example a mixture of potassium and lithium chloride, in electrolytic cells containing an anode, cathode and means for adding metal ions to the bath. Such processes are generally described in, for example, Leone et al., High-PurityTitanium Electrowon from Titanium Tetrachloride, J. of Metals 18 (March 1967); Leone et al., use of Composite Diaphragms in the Electro-winning of Titanium, Bureau of Mines Report RI 7648 (1972) and US Patent No. 2,789,943, No. 2 9,43,032 and No. 3,082,159. In these processes satisfactory titanium is generally obtained, for example, by bubbling titanium tetrachloride gas directly into a molten lithium chloride-potassium chloride catholyte while reducing the titanium ion and depositing metallic titanium on

the cathode and evolution of chlorine at the anode. However, improved means for introducing or feeding a polyvalent metal into the salt-melt bath in the electrolysis cell are also required.

A new and improved device was now developed for introducing an ionizable metal compound into an electrolyte. in an electrolysis cell. Ionizable metals are here polyvalent metals with at least two valence states. This supply device or supply cathode comprises a supply line with at least one inlet and at least one outlet suitable for introducing a metal compound from a source for this into the electrolyte. The feed cathode includes a member which surrounds and substantially completely encloses at least the outlet of the wire. The enclosing member is formed at least in part by an electrically conductive perforated body through which an electrolyte and ions of a polyvalent metal compound can pass.

During operation, the ionizable compound of polyvalent metal is passed through the line and into the electrolyte. After the metal compound has entered the electrolyte,

or mixed with it, it is believed to be dissociated into ions by the metal compound. Preferably, the enclosing member limits the agitation caused by introducing a gaseous metal compound and/or an inert dispersing gas into the feed cathode. ■ An electric current source is connected at least to the pierced member applying a sufficient negative charge to reduce the metal ions from

a higher to a lower valence state.- Electrolyte containing dissolved ions from the metal compound passes through the perforated body and into a. deposition cathode chamber and to the deposition cathode, where solid metal is deposited. In general, the metal ions are believed to be reduced from a higher to a lower valence state within or fairly close to the feed cathode.

More specifically, the invention relates to a metallic diaphragm adapted to separate an anode chamber from a cathode chamber in an electrolysis cell containing a molten salt bath for the electrolytic production of multivalent metals, comprising a perforated member with a diaphragm coefficient between zero and approx. 0.5 and a flow coefficient in the range 0.1-25, where at least the surface of the perforated member consists of a metal which is resistant to the corrosive environment in the cell.

The present invention further relates to an electrolysis cell for the production of multivalent metals in a molten salt bath, where the cell comprises a body which is arranged to contain the bath, means for separating the bath from the surrounding atmosphere, an anode chamber arranged in said body, means for to remove a gas from the anode chamber, a deposition cathode chamber• arranged in said body, at least one anode, which is arranged to be at least partially immersed in the bath, arranged in the anode chamber, at least one deposition cathode, which is arranged to being at least partially submerged in the bath, provided in the cathode chamber, an electrical energy source connected to the anode and the cathode, at least one feed means adapted to supply polyvalent metal ions to the bath, and at least one metallic perforated diaphragm resistant to the corrosive environment in the container and arranged to be at least partially submerged in the bath as a separation between the anode chamber size and the cathode chamber, which diaphragm has a diaphragm coefficient between 0 and about 0.5 and a flow coefficient in the range 0.1-25 and is electrically isolated from electrical sources outside the anode

and the cathode chambers.

The present invention also relates to a method for the production of multivalent metals in an electrolytic cell which has at least one anode placed in an anode chamber,

at least one cathode placed in a cathode chamber, and a supply device for feeding a compound of polyvalent metal into a molten salt bath in the cell, which method

is characterized by the following steps: A perforated diaphragm where at least one surface is resistant to the environment in the cell is inserted into the cell as a separation between the anode chamber and the cathode chamber, which diaphragm has a diaphragm coefficient between 0 and approx. 0.5 and a flow coefficient in the range 0.1-25, the diaphragm is electrically isolated from electrical sources outside the anode and cathode chambers, and an electromotive force is applied between the anode and cathode.

Furthermore, the invention relates to an input cathode device adapted for use in an electrolytic cell containing a molten salt bath, which cathode device comprises an input line for supplying a compound of multivalent metal to the cell, which input line has at least one outlet through which the metal compound can pass from a source thereof to the molten salt bath in the cell, and a member which surrounds and substantially completely encloses at least the outlet of the conduit, which member is formed at least in part by an electrically conductive perforated body through which ions of the polyvalent metal compound and the electrolyte can pass.

The present invention further relates to a method for determining in advance the suitability of the metallic diaphragm for use in an electrolytic cell, characterized by the following steps: (a) a known direct current ignition is applied between a primary anode and a primary cathode immersed in a test cell containing a electrolyte, (b) one measures an electrical property over a predetermined part of the electrolyte with two measuring electrodes. located between the primary anode and the primary cathode and communicating with the predetermined portion of the electrolyte via a first salt bridge and a second salt bridge with the mouths at a predetermined distance from each other, (c) a metallic diaphragm is inserted into the solution between the measuring electrodes , the electrolyte having such a conductivity that the insertion of the diaphragm causes a voltage change between said primary electrodes which is insufficient to convert the metallic diaphragm into a bipolar electrode, and (d) the electrical property over the predetermined part is measured again of the electrode as under (b).

The electrolysis cell according to the invention comprises in principle, in combination, a body designed to contain a molten salt bath and to separate the bath from the surrounding atmosphere. An anode chamber and a deposition cathode chamber are located inside the body separately from each other. The anode chamber and the cathode chamber are separated by at least one perforated metallic diaphragm designed • to be electrically isolated from sources of electrical energy outside the anode chamber and the cathode chamber - and at least partially submerged in the molten salt bath during cell operation. The diaphragm is characterized by the diaphragm coefficient (C^) in the range between zero and approx. 0.5 when the flow coefficient (C^) is in the range 0.1-25. C^ is here according to the definition in inches and Cf in \/. inches per liters per my. per 30 square inches of diaphragm surface. The diaphragm coefficient

can be determined by the procedure described below and indicated by the formula:

"Vd+s" is the voltage (in volts) in an aqueous 0.1 M

sodium chloride solution in a test cell, determined by calomel measuring electrodes communicating with the solution in the test cell via salt bridges with the mouths of the salt bridges spaced 0.75 inch between

silver-silver chloride primary electrodes, but spaced one inch apart, and also separated by the part of the diaphragm which lies between the primary electrodes during operation.

"Id+s" is an electrical current of 0.002 amperes maintained between the primary electrodes in the solution with a diaphragm positioned as

•for V_ .

d+s

"Vg" is a voltage (volt) as it is determined

V"d+s' but without the diaphragm.

"Is" is an electric current of 0.002 ampere maintained between the primary electrodes in the solution as determined for I^+s/ but without the diaphragm.

The flow coefficient is given by the formula:

"h" is a pressure corresponding to 25.4 cm water column at approx. 23.9°C measured upward from the centerline of a circular diaphragm member, with a 30 square inch surface area on a single surface of said diaphragm member, where a water flow measurement through the diaphragm is obtained, and

"F" is the volumetric water flow rate through the diaphragm section in liters per minute at approx. 23.9°C.

The design or size of the diaphragm may make it necessary to use a diaphragm part that is smaller or larger than the above-mentioned 30 square inches when measuring the water flow. When such a smaller or larger diaphragm part is used, F is recalculated so that it represents the water flow through the above described area of 30 square inches.

Said in a slightly different way, the above formula for determining the diaphragm coefficient is assumed in principle to be the total resistance of the diaphragm plus the solution in the test cell minus the resistance of the solution divided by the resistance of the solution. The numerical result of this calculation represents the electrical resistance of the diaphragm expressed as the electrical resistance of 0.75 inch of solution, as the salt bridges are 0.75 inch apart. The aforementioned numerical result can be converted into a term expressed in inches of resolution by multiplying it by 0.75. The diaphragm coefficient represents the electrical resistance of the diaphragm in the test cell. The diaphragm coefficient is also believed to be a measure of the resistance of the solution contained in the pores of the diaphragm.

The electrolysis cell according to the invention further includes at least one anode, which is arranged to be at least partially immersed in the bath, and which is placed in the anode chamber. At least one deposition cathode arranged to be at least partially immersed in the bath is simultaneously arranged in the cathode chamber. A suitable means of removing gases formed at the anode is combined with anode comb size. At least one feed means for supplying a metal-containing starting material, such as an ionizable metal compound, to the bath, and a suitable means for removing metal deposited at the cathode, are combined with the cathode chamber. Furthermore, the anode and the cathode are connected with means for supplying sufficient electrical energy to the anode and the deposition cathode to reduce the metal ions from a higher to a lower valence state and to deposit the metal on the deposition cathode.

The electrolysis cell described here is suitable for the production of titanium and others. polyvalent metals in a molten halide bath. Examples of suitable polyvalent metals which have at least two possible valence levels in the ionized state are

Ti, V, Cr. Mn, Fe, Co, Ni, Y, Zr, Nb, Mo, Ru, Rh, Pd,. Tea, Us,

Ir and Pt. Generally, and preferably, the compound is a halide such as a fluoride, bromide, iodide, preferably a chloride. Although the following description deals with the preferred titanium, it applies to the polyvalent metals in general.

A preferred apparatus and method lies in the extraction of titanium. The titanium compound supplied to the cathode chamber is characterized by the fact that it is at least partially and preferably substantially completely ionizable in the molten salt bath. The titanium ions are reduced from a higher to a lower valence state at the cathode. A gas, such as halogenated chlorine, is evolved at the anode. The gas and the metallic titanium are removed from the cell by suitable means.

Reference is now made to the drawing, which further illustrates the invention. Fig. 1 shows a cross-section through an electrolysis cell for the production of a solid polyvalent metal. Fig. 2 shows a cross-section through another embodiment of the invention. Fig. 3 schematically shows means for measuring the water flow rate through a diaphragm. Fig. 4 schematically shows an apparatus which is suitable for measuring the diaphragm coefficient.

Follow 5 illustrates an embodiment of an input. cathode according to the invention.

Fig. 6 illustrates another embodiment of the invention in combination with an electrolytic diaphragm cell.

The same reference number, marked with a suffix, on

the different drawing figures "represent parts with the same or similar function in the different embodiments.

Fig. 1 shows electrolytic equipment 10 suitable for electrolytic extraction of multivalent metals, preferably titanium, in a molten salt bath from compounds thereof. The following description relates to a preferred apparatus for the electrolytic recovery of titanium; however, the description also applies to the multivalent metal in general.

The molten salt forms a solvent for the titanium compound. Such salts or mixtures thereof can for example be NaCl, LiCl-KCl, LiCl-KCl-NaCl and LiCl-KCl-C.aCl2. When titanium is recovered from titanium tetrachloride, the molten salt conveniently contains a mixture of alkali metal or alkaline earth metal halides, preferably lithium or potassium chloride. A eutectic mixture of the salts used in the bath is advantageous due to the low melting temperature of such mixtures,

The electrolysis equipment 10 includes a container 12 for the molten bath of halide salt and titanium tetrachloride, and

. the container 12 is made of materials which are not significantly damaged by the melt bath. A number of different materials can be used, but the container 12 is usually made of a metal, such as steel or nickel. The container 12 is divided into at least an anode chamber 14 and a deposition cathode chamber 16. Anode chamber 14 and cathode chamber 16 are separated from each other by a porous

metal diaphragm 17. A diaphragm holder 15 can optionally be combined with the diaphragm 17, with the aim of increasing the diaphragm's strength under the conditions used in the equipment 10.

The diaphragm substrate is preferably a metal grid, plate or film with a number of through holes or pores. These pores can, for example, be formed by drilling, punching, weaving or sintering. Generally, and preferably,. the holes in the substrate are of substantially uniform size. The diaphragm 17 is preferably a woven metal wire cloth with mesh sizes corresponding to US Standard Screen Mesh of 50-250 and preferably 100-200, on which sufficient nickel or cobalt has been electrolytically or electroless deposited to give a desired diaphragm coefficient (C ^) and flow coefficient (Cf). The deposited metal preferably mainly consists of cobalt or nickel. Suitable deposition methods are those known in the field, adapted for the production of a visually matt or rough surface by, for example, using a reduced amount of brightening agents in the plating solutions. Table I shows cobalt and nickel solutions for electroless plating which are suitable for use when plating the diaphragm 17.

The diaphragm substrate can for example be iron, such as steel or stainless steel, but it is suitably a metal such as cobalt, nickel or an alloy thereof containing at least approx. 50% by weight cobalt or nickel, and which is resistant to the corrosive environment in the container 10 and retains sufficient strength at predetermined operating temperatures to be able to act as a diaphragm. The diaphragm substrate is preferably of commercially pure nickel.

An anode 18 is placed in the anode chamber 14 and arranged so that it will be at least partially immersed in the molten bath of halide salt during the operation of the electrolytic equipment 10. The material of which the anode 18 is made is resistant to the corrosive effects of the molten salt baths and against the chlorine that forms at the positively charged anode during cell operation. Suitable anode materials are, for example, carbon or graphite. A cathode 20 is placed in the cathode chamber 16 so that it will be at least partially immersed in the molten salt bath during the operation of the electrolysis equipment 10. The deposition cathode 20 is of a material such as carbon or a metal such as, for example, ordinary carbon steel or titanium, on which it can metallic titanium is deposited which can then be extracted.

The cathode chamber 16 also includes a device (not shown) suitable for heating and for keeping the contents of the equipment 10 at a desired temperature during heating or cooling,

.and one feeding device 22 specifically for introducing a titanium-containing material into the molten salt bath during operation of the equipment 10. During operation, titanium tetrachloride is fed from a container

■24 through a line 26 to the feeding device 22, where the titanium tetrachloride passes through a number of openings or holes 28 in the device 22 and into the molten salt bath in the cathode chamber 16.

The container 12 is provided with covers 30, 30a and 30b, which give access to the anode 18, the cathode 20 and the feeding device 22. The covers 30, 30a and 30b are preferably removably attached to the container 12, so that it becomes possible to use a regulated atmosphere in the container 12 and prevent so much of the surrounding atmosphere, especially nitrogen, oxygen, carbon dioxide and water vapour, from entering the container 12 during operation that the efficiency of the process is significantly reduced. Preferably, the covers 30, 30a and 30b are capable of virtually completely excluding oxygen from the chambers 14 and 16.

■The cover 30a is also arranged to provide a means of removing the metallic titanium from the cathode chamber 16 after a solid titanium metal has been deposited on the cathode 20.

During operation, it is ensured that the atmosphere in the electrolysis cell 10 has a content of atmospheric gases below predetermined limits. The presence of significant amounts of oxygen, particularly amounts approaching the content of the surrounding air, is undesirable, as the cell's efficiency, the cell's operating life and the quality of the titanium product are thereby reduced. Accordingly, it is preferred that oxygen and other reactive gases be substantially completely excluded from the chambers 14 and 16. The cover 30a is arranged to exclude oxygen and to provide a means of removing the titanium metal from the cathode chamber 16 after solid titanium is deposited on the cathode 20.

Chlorine gas formed at the anode 18 flows to a condenser or chlorine container (not shown) from the anode chamber 14 through a chlorine outlet device or line 32.

A device that supplies electrical energy, such as a generator or rectifier 34, supplies sufficient electrical energy to the equipment 10 to reduce titanium ions having a valence of +4 to a lower valence state, depositing metallic titanium on the negatively charged deposition cathode 20 and liberating chlorine at the positively charged anode 18. The anode 18, the cathode 20, the feed device 22 and the diaphragm 17 are electrically isolated from the container 12 by insulators 35. Furthermore, the diaphragm 17 is electrically isolated from sources of electricity outside the anode chamber 14 and the cathode chamber 16, such as the electrical equipment that is

connected to the anode 18 and the cathode 20. The diaphragm 17 is in other words. placed in the container 12 and working in the equipment 10

without being connected to electrical wires that give the diaphragm an electrical charge.

The container 12 may optionally be provided with device means for the diaphragm, such as flanges 36 at a suitable distance from each other to form a holding device in which the diaphragm 17 can be removably mounted. Should it become necessary to replace the diaphragm 17 the operation of the embodiment shown in fig. 1, another diaphragm (not shown) can be placed next to the diaphragm 17 in the unused flanges 36 before the diaphragm 17 is removed. If desired, more than one diaphragm can be used at the same time with the help of the flanges 36. Alternatively, the flanges 36 can be used to hold at least one filter (not shown) in at least the cathode chamber 16 and possibly the anode chamber 14 to prevent mechanical damage to or clogging of the diaphragm 17 with solids contained in the catholyte or anolyte.

Fig. 2 illustrates a preferred embodiment of an electrolysis cell device 10a, where an externally heated and/or cooled container 12a is arranged to hold a potassium chloride-lithium chloride-titanium dichloride-titanium trichloride-containing catholyte in a cathode chamber 16a and a lithium chloride-potassium chloride electrolyte in an anode chamber 14a. Anode chamber dimension 14a is separated from cathode chamber 16a by. a porous woven diaphragm 17a surrounding an anode 18a spaced therefrom. In order to increase the diaphragm's lifetime or service life, the distance between the diaphragm and the anode is preferably allowed to be at least 1/4, preferably between 1/4 and 1^ times the anode's diameter and most preferably substantially equal to the anode's diameter. Two deposition cathodes 20a and a feed cathode 22a for titanium ion-containing material are arranged in the cathode chamber 16a at intervals and at a distance from the diaphragm 17a. The container 12a is also electrically isolated from the diaphragm 17a and the various electrically charged components of the device 10a.

The container 12a is preferably mainly gas-tight, so that atmospheric gases do not penetrate into the anode chamber 14a and/or the cathode chamber 16a. A better control of the operation of the cell device 10a in a mainly inert atmosphere is achieved by means of an inlet 37 for the supply of protective gas to the closed container 12a. For titanium, the atmosphere is a gas such as argon or helium, which is essentially inert to the electrolyte and titanium at the normal operating temperatures. When a lithium chloride-potassium chloride electrolyte is used in combination with titanium tetrachloride, the operating temperature is usually within the range between the eutectic temperature of the salt mixture (about 348°C) and about 650°C, preferably between 475 and 575°C. The operating temperature will of course vary according to the melting point or melting range of the electrolyte used.

To facilitate removal of the anode 18a, deposition cathodes 20a, or feed cathode 22a when they are to be replaced or examined, it is preferred to provide gas-tight chambers, such as airlocks 38, 38a and 38b, so that the cathodes and/or anode can be removed without significant contamination. of the atmosphere in the anode chamber 14a or the cathode chamber 16a with reactive atmospheric gases. Means, such as valves 40,

suitable for sealing the anode chamber 14a and the cathode chamber 16a against the external atmosphere serves to prevent reactive gases from entering the container 12a and contaminating the atmosphere therein. The valves 40 are arranged to be able to close and seal the airlocks 38, 38a and 38b by a sliding movement when the anode, cathodes or diaphragm is removed from or inserted into the container 12a. The use and operation of such valves and airlocks is in and of itself well known.

A line 32a for removing the chlorine gas that is formed is at least partially located within the anode airlock 38b. The deposition cathode airlocks 38a can be used to remove metallic titanium from the cathode chamber 16a.

A valence electrode 42 is arranged to be at least partially immersed in the molten halide electrolyte for determining the average valence of the titanium ions in the electrolyte during operation of the cell device 10a. The valence electrode 42 may be arranged to be connected to a supply source 24a for titanium tetrachloride and a dosing device for titanium tetrachloride, for. example. a pump 44, to regulate the titanium ion concentration and thus the average titanium ion valence, in the cathode chamber 16a. The dosing pump 44 is arranged to provide a regulated supply of titanium tetrachloride to the feed cathode 22a through line 46, so that the titanium ion concentration is kept at a predetermined level.

Preferably, a temperature regulator 47 is provided to keep the electrolyte in anode chamber 14a and cathode chamber 16a at predetermined desired temperatures. The temperature regulator 47 can effect cooling or heating of the electrolyte as required, which can be done by means known per se, for example by means of air, electricity, gas or oil.

During the operation of the cell device 10a, unwanted oxides, nitrides and other solid materials can accumulate in the container 12a, such as the waste material that professionals in the field usually call sludge. A sludge removal device, such as a line with valve 48, can be used for manual or mechanized removal of the sludge without large losses of electrolyte from the cell device 10a.

The design of the diaphragm 17a is very important in the described apparatus. It is necessary that the pores or openings in the diaphragm 17a are large enough not to be clogged with, for example, particles of metallic titanium, titanium oxide or sludge. Furthermore, the pores are sufficiently small to prevent significant quantities of the molten salt bath containing the titanium ions from passing through and into the anode chamber 14a from the cathode chamber 16a. At the same time, the openings are preferably of sufficient size to allow the passage of a sufficient amount of lithium chloride-potassium chloride electrolyte from the cathode chamber 16a to the anode chamber 14a so that a desired bath level is maintained in the anode chamber 14a. A metal diaphragm with an electrolytically or electroless deposited coating of preferably cobalt on a preferred nickel substrate has been found to meet the above requirements. The plated diaphragm preferably has a Cd of 0.1-0.5, preferably 0.1-0.4, when Cf is 0.1-25. Diaphragms with a lower C 2 , for example 0.003, have, however, been found to be satisfactory for the production of titanium and fall within the scope of the invention. C f is preferably 0.1-8,

■preferably 0.2-1.

By using the described apparatus, and in particular the porous diaphragm with predetermined C-, and C^, it has been found that a polyvalent metal, such as titanium, can be produced without it being necessary to regulate the diaphragm's pore size during electrolysis. As the diaphragm preferably has a mesh-like metal substrate with an adherent metal coating, it can be easily stored before use and is more resistant to mechanical failure than diaphragms containing ceramic materials.

In fig. 3 schematically shows an apparatus for measuring the volumetric flow rate of water through a diaphragm. Water with a temperature of approx. 75°C is supplied from a source 50 to a diaphragm 52 through a conduit 54. The water flow rate is sufficient to maintain a water level in an upwardly extending conduit 56 at a distance of 10 inches from the axis A of conduit 54 to the upper surface of the water in line 56. The upper end of line 56 is open to the atmosphere. Maintaining such a level in line 56 ensures that the average pressure across the diaphragm 52 being tested is approx. 25.4 cm water column. The volume of water flowing through a part of the diaphragm 52 corresponding to 30 square inches is appropriately measured in, for example, a container 58. The measured flow rate in liters per minute, is used to determine the flow coefficient C^.

Reference is now made to the test apparatus or cell in fig. 4. It has so far been difficult and sometimes impossible to decide in advance whether a given diaphragm would be suitable for use in an electrolysis cell. It is known that the material from which the diaphragm is made should be substantially non-reactive,

i.e. physically and chemically inert to the electrolyte in the electrolytic cell; however, a means of accurately determining in advance the effect of a given diaphragm's design and surface properties on the efficiency of a cell is not generally known.

It is therefore highly desirable to provide a means of determining whether a diaphragm will be effective in a cell, before the diaphragm is inserted into the electrolysis cell.

A method of measuring whether a diaphragm

is suitable for use in an electrolytic cell was now developed. The method consists in applying a known direct current voltage between a primary anode and a primary cathode which is immersed in a test cell containing an electrolyte. An electrical property over a predetermined portion of the electrolyte is measured with two measuring electrodes placed between the primary anode and the primary cathode bg which communicate with the predetermined portion of the electrolyte via a first salt bridge and a second salt bridge at a predetermined mutual distance. The diaphragm to be examined is inserted between the measuring electrodes, and the electrical property over the predetermined portion of the electrolyte is measured again. The change over the electrolyte due to the insertion of the diaphragm can then be determined.

In the present context, diaphragm means a porous barrier placed between an anode and a cathode in an electrolysis cell. The diaphragm can, for example, be made of asbestos, conductive porous plate material or mesh-like material, sintered porous material and similar materials.

Then the effectiveness of the diaphragm at least

partly depends on both the porosity of the diaphragm and surface properties such as roughness, a flow-through test alone (ie measuring the amount of liquid passing through a known

area of the diaphragm during a certain time) to determine the porosity is usually not an accurate measure of future diaphragm efficiency under the electrolysis conditions.

The present method for determining, for example, the electric current passing through the pores of the diaphragm,

has been found to be dependent on both the physical porosity of the diaphragm and the surface property. The measurements of electrical voltage, resistance and/or amperage carried out according to the described method have surprisingly been found

• to give an accurate indication of diaphragm efficiency. The procedure can be used, for example, to examine i

the extent to which the diaphragm is suitable for electrolytic purposes, or as a means of periodically or continuously monitoring

• quality control during production and/or during operation

of such diaphragms.

In fig. 4 are primary electrodes, such as a primary anode 60 and a primary cathode 61 immersed in an electrolyte 6 2

and connected to an energy source 64. Suitable electrolytes are compatible with the electrodes 60 and 61 and with a diaphragm 66 and have sufficient electrical conductivity to accurately determine the electrical effect of the insertion of the diaphragm 66 into the electrolyte 62. The electrodes 60 and 61 and the electrolyte 62 is chosen with a view to obtaining a cell with a reversible electrolytic reaction. Furthermore, when a metallic diaphragm is examined, the conductivity of the electrolyte 62 should preferably be such that the insertion of the diaphragm 66 into the electrolyte 62 will cause an insufficient voltage change between the electrodes 60 and 61 to result in the diaphragm 66 becoming a bipolar electrode. Examples of generally satisfactory electrolytes include aqueous inorganic salt solutions or acid solutions, such as solutions of chlorates, chlorides, nitrates and sulfates of metals. Suitable metals are, for example, alkali metals, alkaline earth metals and transition metals and preferably the alkali and alkaline earth metals, such as Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and. Ba. The materials used as material for the primary electrodes are those which are generally known to be able to be used as electrodes, for example graphite, Ru, Rh, Pd, Ag, Os, Ir, Pt and Au. Silver-silver chloride electrodes have been found to be particularly suitable for use as primary electrodes and are preferred.

The primary electrodes 60 and 61 are suitably placed inside holding means 68 and 69 which are essentially electrically non-conductive, so that the surface 65 of the electrode 60 is held at a predetermined distance, for example approx. 2.5 cm, from the surface 67 of the electrode 61. The holding members 68 and 69 can, for example, be made of a methyl acrylate plastic and arranged so that essentially the entire electric current will pass between the electrodes 60 and 61' through the diaphragm 66 when the diaphragm is removable attached to.the holding means.

Two calomel measuring electrodes 70 and 72 are connected to the electrolyte 62 via a first salt bridge 74 and a second, salt bridge 76. The mouths 78 and 80 and the salt bridges 74 and 76 project through the holding members 68 and 69 in a predetermined position between the electrodes 60 and 61 and communicates with the electrolyte 62. The mouths 78 and 80 are placed at a predetermined mutual distance, for example 1.9 cm, counted from the center lines B and C of the mouths.

The measuring electrodes or auxiliary electrodes 70 and 72<.> which are suitable for use in the present invention are known in and of themselves. For example, calomel, cadmium, hydrogen, mercury electrodes and the like can be used as measuring electrodes.

In the implementation of the present invention, a direct current voltage is applied between the primary anode 60 and the primary cathode 61, whereby a constant current strength is obtained between the electrodes.

The electromagnetic force or voltage applied between the electrodes 60 and 61 should produce a voltage drop

through the electrolyte which is less than the potential required for splitting the electrolyte 62. When the electrolyte is, for example, an aqueous solution of NaCl, the voltage drop across the electrolyte should in any case be less than the splitting potential for H20.

An electrical property, preferably the voltage, over the predetermined distance between the salt bromide mouths 78 and 80 is measured using the measuring electrodes 70 and 72. The electrolyte's resistance is determined by dividing the measured voltage between the measuring electrodes 70 and 72 by the known current strength.

The diaphragm 66 is placed in the electrolyte 62 between the electrodes 60 and 61 and the salt bromide orifices 78 and 80, whereby the electrical resistance between the measuring electrodes over the predetermined distance between the orifices 78 and 80 is changed. As previously mentioned, the diaphragm 66 is placed in contact with the holding member 68 in such a way that the greatest possible

current strength through the area of the diaphragm that is delimited by the holding member 68, and a minimum possible current strength through any openings at the interface between the surface of the holding member 68 and the diaphragm 66 or around the edges of the diaphragm 66.

The diaphragm 66 is placed in the electrolyte 62 between the electrodes 60 and 61 and the salt bromide openings 78 and 80 of the measuring electrodes 70 and 72, whereby the electrical resistance between the measuring electrodes changes. At a constant known current, the change in voltage drop across a predetermined part of the electrolyte 62, measured by means of measuring electrodes 70 and 72, be a quantity characteristic of the porosity and surface properties or efficiency of the diaphragm 1 a diaphragm electrode light cell. Such diaphragm cells are suitable for the electrolytic production of, for example, chlorine from sodium chloride solutions or, preferably, a polyvalent metal, such as titanium from titanium tetrachloride.

The present method can be used to determine the uniformity of a diaphragm using primary electrodes of such size and shape that the direct current provided between them only passes through a known area of the diaphragm. An electrical property, such as electrical voltage,

resistance, or amperage, between the primary electrodes can be measured over a predetermined portion of the electrolyte before and after insertion of the diaphragm between the primary electrodes. After each measurement, the diaphragm is moved in relation to the primary electrodes, so that the subsequent measurement concerns a different part of the diaphragm. A comparison between the results of two or more measurements will then provide information on the uniformity or lack thereof in terms of the permeability and surface properties of the diaphragm. The experiments can be carried out at any temperature or pressure provided that these are kept constant.

The method described here has been found

to be satisfactory for porous metal cloth, plate or mesh diaphragms and particularly well suited for porous woven metal cloth with a metal coating.

The following examples will further illustrate the method according to the invention.

Example 1

Using an apparatus substantially as shown in the figure, the suitability of a nickel-plated, woven nickel cloth, diameter approx. 5.1 cm and length approx. 25.4 cm, front

application as an electrolytic cell diaphragm defined below

. use of an aqueous 0.1 molar sodium chloride electrolyte (the sodium chloride had a purity of 99.5% by weight and was dissolved in distilled water), two rectangular silver-silver chloride primary electrodes with dimensions approx. 3.2 cm x approx. 1.3 cm x approx. 1.6 mm and a mutual distance of 2.54 cm, as well as two standard calomel electrodes. which were physically connected between the primary electrodes via salt bridges for measuring a voltage applied over a distance or length in the sodium chloride solution of approx. 1.9 cm. The silver-silver chloride electrodes were mounted in a frame of methyl acrylate plastic designed to allow insertion of the diaphragm

between the electrodes.

A direct current voltage was applied across the primary electrodes so that a current of 2 milliamps (mA) was obtained between the primary electrodes. The voltage and direct current between the measuring electrodes were determined before and after the insertion of the diaphragm between the electrodes. The experiments were carried out at a constant room temperature (approx. 20°C) and a pressure of one atmosphere.

The voltage figure across said portion of the sodium chloride electrolyte was found to be 68 millivolts (mv), and

it was found that the amperage was. 2 milliamperes (mA) before insertion of the diaphragm. The voltage drop between the measuring electrodes increased to 93 millivolts after the diaphragm was inserted into the test cell; the amperage was kept constant at 2 milliamps.

The voltage increase. of 25 millivolts was calculated by ordinary methods to be equivalent to an increase in the resistance of the test cell of 12.5 ohms, or 0.70 cm of sodium chloride electrolyte between the electrodes.

Several diaphragms of the same material were tested as described above and used in an electrolysis cell for the production of titanium metal from titanium tetrachloride. The diaphragm coefficient (C^) or diaphragm electrolyte equivalent (unit length) was determined using the formula below and compared for the satisfactory and unsatisfactory diaphragms. A satisfactory range for the diaphragm coefficient (C^) was thereby determined.

The diaphragm coefficient is represented by the formula:

V^+s- is the voltage drop (millivolts) measured across a predetermined portion of an electrolyte using measuring electrodes that communicate with the electrolyte via salt bridges with mouths

■which have a predetermined mutual distance , (D), a diaphragm being placed between the salt bridge mouths during operation.

Id+s- is the electric current (milliamps) measured between the primary electrodes in the electrolyte with a diaphragm placed as for

W

■V is the voltage drop (millivolts) measured under the same conditions as for v^+s,

but without the diaphragm.

I - is the electric current (milliamps) measured between the primary electrodes in the electrolyte as for I(j+S/ but without the diaphragm.

D - is the predetermined distance between the salt bridge mouths.

Examples 2-4

In essentially the same way as described in example

1, the coefficients (C^) for other metal cloth diaphragms were determined. The experimental conditions and results are indicated in Table II. Additional metal cloth diaphragms were investigated by the present method using (a) 0/01 molar I^SO^ as the electrolyte and graphite as the primary electrodes and (b) 0.01 molar NaCl as the electrolyte and silver-silver chloride as the primary electrodes. Satisfactory results were obtained.

Examples 5-42

Metallic titanium with a purity of approx. 99.9% by weight

was produced from titanium tetrachloride (TiCl^) in an electrolysis cell corresponding to the one shown in fig. 2. The electrolysis equipment included a substantially cylindrical container, of low carbon steel, which cylinder had an outside diameter of 45.7 cm and a height of 55.9 cm. An essentially cylindrical diaphragm with a diameter of 4.8 cm and a length of 16.5 cm and

an enclosed lower end was arranged substantially symmetrically around a compact graphite anode with a diameter of 1.9 cm and a length of approx. 45.7 cm. About. 15.2 cm of the length of the anode was immersed in a molten lithium chloride-potassium chloride bath with an approximately eutectic composition corresponding to approx. 55% by weight LiCl and approx.

45 wt% KC1.. The diaphragms consisted of commercial metal cloth

pure nickel that was electrolytically or electroless plated with a sufficient amount of cobalt or nickel to give the desired Cd and Cf ^see tabe11 HI-and IV)- The plating was carried out in plating solutions suitable to give a rough and matt or low light-reflecting surface. In order to extend the diaphragm's service life, a distance between the diaphragm and the anode was chosen within the range between 1/4 and 1^ times the anode diameter.

A deposition cathode consisted of a mild steel rod with a diameter of 2.54 cm and a length of 19 cm. A feed device or feed cathode was provided for supplying gaseous TiCl 2 to the molten electrolyte. The feed cathode consisted of a stainless steel tube with a cylindrical,

100 mesh cloth of iron or nickel wire arranged substantially symmetrically around the tube at a distance therefrom, which cloth was electrolytically or electroless plated with cobalt, iron or nickel. The lower part of the cloth was enclosed. The plated feed cathode cloth had a C^ of 0.1-0.6 and a C^ of 0.2-30.

During the experiment, liquid TiCl^ was pumped into the feed cathode, where it evaporated and was reduced to TiCl^ and TiCl 2 as it passed through pores in the feed cathode and into the molten catholyte. Electric current was supplied to the feed cathode and the anode and the cathode whereby chlorine was evolved at the anode and titanium metal was deposited on the deposition cathode. The chlorine gas was continuously removed from the anode chamber through a line arranged through a cover of the electrolysis cell. Titanium was periodically removed from the cathode by first removing the deposition cathode from the electrolysis cell and then scraping the deposited sponge titanium from the cathode. The cathode was then reinserted into the cell. The atmosphere in the anode chamber and the cathode chamber was kept substantially inert by continuous supply of sufficient amounts of argon gas to the chambers to maintain a positive pressure therein relative to the atmosphere surrounding the cell.

Tables III and IV show the individual process parameters together with the titanium current yield and the titanium hardness obtained in examples 5-42. It will be seen from Tables III and IV that titanium metal with a low hardness and therefore a high purity can be efficiently produced by the described method.

In substantially the same manner as described in Examples 5-42, titanium was satisfactorily prepared using diaphragms having a C. of 0.003 and a C.sub.3

on 1.1.

Fig. 5 shows a metal compound feed cathode for use in an electrolytic cell for the electrolytic deposition of a metal onto a negatively charged cathode. The feed cathode comprises a supply line, such as a tube 80, arranged for the supply of an ionizable compound of multivalent metal.

A member 84 is arranged generally surrounding an opening 82 in the tube 80 for introducing a metal compound, such as

preferred titanium tetrachloride (TiCl3), in an electrolyte, such as a molten mixture of potassium chloride and lithium chloride. The enclosing member 84 represents at least a portion of an electrically conductive through-hole member 86 suitable for introducing ions of the metal compound into the molten electrolytic bath from a metering electrode chamber, such as a generally annular opening 88 formed by the outer surface of the tube 80 and the inner of the enclosing member 84, to a catholyte which is present in an adjacent deposition cathode chamber (not shown). Mainly gas-impermeable elements 90 join the upper and lower end portions of the member 86.

The enclosing member 84 is arranged generally coaxially around the tube 80 and is fixedly mounted. An electrically insulating element 92 can optionally be placed to separate the pipe 80 from the surrounding member 84, so that the pipe 80 is electrically isolated from the pierced member 86 when a negative voltage is applied to the member 86 from a power source 94.

When using the embodiment of fig. 5, the feed cathode is arranged in an electrolysis cell in such a way that the electrically charged part is at least partially, and preferably

practically completely, projecting below the surface of a molten halide electrolyte. An ionizable metal compound, such as titanium tetrachloride, flows or is pumped from a reservoir 24, which is shown in FIG. 1, through the line 80 and into the electrolyte feed chamber 88 through the opening 82. The power source 94 is connected so that a negative charge is applied and makes the perforated member 86 cathodic. The electrically negative member 86 at least partially reduces the titanium ions in and around the feed cathode from a higher valence to a lower valence.

By using the perforated member 86 with pores of sufficient size to pass through the molten electrolyte without transferring a significant amount of physical turbulence from the annular opening 88 (resulting

from, for example, gaseous TiCl^ coming into the chamber. Into the cathode chamber which surrounds the feed cathode, improved utilization of TiCl^ is achieved compared to the previously known processes.

Fig. 6 shows another embodiment of the invention, including an electrolysis cell 100 with a container 102 and a cover 104. A metal deposition cathode 106 is arranged in the cathode chamber 108 and is separated from a positively charged anode 110, which is arranged in an anode chamber 112 , by a porous diaphragm 114. An input cathode 116 is arranged in the electrolysis cell 100

and arranged to be at least partially immersed in the catholyte in the cathode chamber 108. The feed cathode 116 includes a supply line 80a of a material which makes it suitable for being immersed in the molten salt catholyte, through which line TiCl 2 can be introduced. The catholyte is preferably a mixture of molten halides, for example chlorides of lithium and potassium. Titanium tetrachloride may flow through line 80a simultaneously with an inert gas, such as argon. The inert gas causes the mixture titanium tetrachloride, or a solid metal compound,

in the catholyte. A through-hole member 86a substantially completely surrounds a lower outlet 82a in the conduit 80a. The member 86a is carried by and is electrically connected to a support member 118, which is optionally arranged generally coaxially at a distance from the line 80a.

A heater, for example an electric heating element 120, can optionally be inserted between the line 80a

and the carrier 118 for heating the gases that flow through the carrier 118 or the line 80a, and to prevent freezing of the electrolyte in the vicinity. The carrier body 118

is provided with a gas outlet 122 which allows an excess of gases, such as argon, which. does not dissolve or dissociate in the electrolyte, and flow through the annular space 124 to a suitable container (not shown). The support member 118 is optionally provided with a substantially gas impermeable element 90a suitable to be at least partially immersed in the electrolyte to prevent excess gas from entering the atmosphere inside

in the electrolysis cell 100. Conveniently, practically the entire member 86a is immersed in the electrolyte.

The perforated body 86a is characterized by

an electrical coefficient (C^) which is greater than 0 and goes up to approx. 1, and which is preferably in the range 0.1-1, when the flow coefficient (C^) is in the range 0.1-300. The coefficient of the perforated body is determined by the same method as described for determining the diaphragm coefficient.

The perforated member 86a is, for example, a sintered plate, a cloth, sheet or film with a large number of substantially uniform holes or through pores. These can, for example, be formed by drilling, punching, weaving etc.

The perforated member 14a is preferably a woven metal wire cloth with a US Standard Screen Mesh of 50-250, preferably 100-200, on which a sufficient amount of a material such as cobalt, iron or nickel has been deposited electrolytically or electroless to achieve of a desired C-, and C^. Proprietary deposition methods are methods known in and of themselves which give a visually matt or rough surface by using, for example, a reduced amount of brightening agents in the plating solutions. Satisfactory plating of sheets of carbon steel or commercially pure nickel where the mesh size corresponds to 100 or

200 mesh, has been carried out using the following solutions:

The substrate in the perforated member may be of a material such as iron, including steel and stainless steel; cobalt or nickel or an alloy thereof containing at least approx. 50% by weight cobalt or nickel, and which is resistant to the environment in the electrolysis cell 100 and maintains a desired physical strength at the cell's operating temperature.

The design of the perforated member 86a is important in the described apparatus. It is necessary that the pores or openings in the member 86a are large enough so that they are not clogged with, for example, significant amounts of particulate metallic titanium, other multivalent metals, titanium oxide or sludge. Furthermore, the pores should have a sufficient area to minimize and preferably practically completely prevent turbulence in the feed cathode 116 from entering the cathode chamber 108. At the same time, the pores are preferably of a sufficient size to allow the passage of a sufficient amount of a preferred lithium chloride-potassium chloride- electrolyte from the cathode chamber 108 and into the feed cathode 116, so that the desired bath level is maintained. The plated perforated member preferably has a C-, of 0.1-0.6 when C^ is 0.1-300. C f is preferably 0.2-30, more preferably 0.2-8.

The method of operation when using the feed cathode 116 in fig. 6 is mainly as described for fig. 5, with the addition that the electrical current source is appropriately connected to the deposition cathode 106, so that it receives a predetermined negative charge, and to the -anode 110 so that it receives a predetermined positive charge. When titanium tetrachloride is fed into the cathode 116, metallic titanium is deposited on the cathode 106, and chlorine is evolved at the anode 110 and flows upward to a chlorine container (not shown). Preferably, no significant permanent deposit of metallic titanium should be obtained on the through-hole member 86a.

The following examples will further illustrate the invention.

Examples 43-49

Metallic titanium with a purity of approx. 99.9% by weight

was prepared from TiCl 2 in a low carbon steel electrolysis cell using a feed cathode substantially like that of FIG. 5 and an anode which was separated from the cathode by et. diaphragm. The electrolysis equipment included a mainly cylindrical shaped container with an outer diameter of approx. 45.7 cm and a height of approx. 55.9 cm. A mainly cylindrical designed diaphragm, diameter approx. 4.8 cm and length approx. 16.5 cm, with a closed lower end was arranged essentially symmetrically around a compact graphite anode with a diameter of approx. 1.9 cm and a length of approx. 45.7 cm. About. 15.2 cm of the anode length was immersed in a molten lithium chloride-potassium chloride bath of approximately eutectic composition. The perforated bodies consisted of 100 mesh woven fabric of either an iron-alloy rich or commercially pure nickel, the fabric being electrolytically or electroless plated with a sufficient amount of cobalt, iron or nickel to give the C^ and C^ indicated in tableau VI.

Operation of the electrolysis cell in Examples 43-49

by applying one electrical voltage to the anode and cathode resulted in a satisfactory titanium metal product using feed cathodes with woven fabric perforated members having properties as indicated in Table VI.

Titanium tetrachloride was continuously pumped into the feed cathode, where it was ionized and then passed into the cathode chamber through the pores of the feed cathode's perforated woven fabric body. Turbulence inside the feed cathode, caused by the TiCl 2 entering the electrolyte, was satisfactorily confined to the feed cathode. The chlorine developed at the anode, and the titanium deposited on the cathode, were removed from the cell.

It will be understood from the above description that the invention has many embodiments which can be easily varied and modified in other ways than those described above.

Claims (2)

1" Method for testing the suitability of a metallic diaphragm for use in an electrolytic cell, characterized by the following steps: (a) applying a known direct current voltage between a primary anode and a primary cathode immersed in a test cell containing an electrolyte; (b) measuring an electrical property over a predetermined portion of the electrolyte with two measuring electrodes located between the primary anode and the primary cathode and communicating with said predetermined portion of the electrolyte via a first salt bridge and a second salt bridge whose mouths arranged at a predetermined distance from each other; (c) a metallic diaphragm is inserted into the solution between the measuring electrodes, which electrolyte has such a conductivity that the insertion of the metallic diaphragm causes a voltage change between said primary electrodes which is insufficient to convert the metallic diaphragm into a bipolar electrode; (d) the electrical property is again measured over said previously determined part of the electrolyte as under (b). 2. Method according to claim 1, characterized in that the electrolytic dissolution is an approx. 0.1 molar aqueous sodium chloride solution. <3.> Method according to claim 1 or 2, characterized in that the electrical property that is measured is voltage, and that the voltage above the aforementioned certain part of the electrolyte is lower than that required to split the electrolyte. 4. Method according to claim 1 or 2, characterized in that the electrical property that is measured is resistance or amperage. <5*> Method according to one of claims 1-4, characterized in that one moves the diaphragm in relation to the measuring electrodes so that electric current will pass through another part of the diaphragm, and re-measures the voltage across the predetermined part of the sodium chloride the solution as in step (b), whereby comparable voltage .measurements are obtained which provide information on the diaphragm's uniformity and suitability for use in the electrolysis cell. ^* Method according to one of the claims 1- <5>, characterized in that a silver electrode is selected as the primary anode and a silver chloride electrode is selected as the primary cathode and calomel electrodes are selected as the measuring electrodes. <7> » Method according to one of claims 1-6, characterized in that the diaphragm is placed between the measuring electrodes so that essentially the entire current between the primary anode and the primary cathode will pass through the diaphragm, and the diaphragm is removably attached to an essentially electrically non-conductive holding device having the primary anode and the primary cathode arranged therein. ^* Method according to one of claims 1-6, characterized by placing the diaphragm separately from the primary anode and the primary cathode and between the measuring electrodes, so that essentially the entire current between the primary anode and the primary cathode will pass through the diaphragm.