EP3033443B1

EP3033443B1 - Molten salt electrolysis apparatus and process

Info

Publication number: EP3033443B1
Application number: EP14836690.9A
Authority: EP
Inventors: David Steyn Van Vuuren; Dewald TERBLANCHE; Eugene SWANEPOEL
Original assignee: Council for Scientific and Industrial Research CSIR
Current assignee: Council for Scientific and Industrial Research CSIR
Priority date: 2013-08-16
Filing date: 2014-08-15
Publication date: 2018-03-28
Anticipated expiration: 2034-08-15
Also published as: EP3033443A2; WO2015024030A3; US20160215405A1; WO2015024030A2

Description

Field of the invention

The invention relates to alkali and alkaline earth metal production through electrolysis of molten chloride salts thereof.

Background to the invention

The production of alkali metals such as metallic lithium and sodium and an alkaline earth metal such a magnesium is very energy intensive, resulting in very large consumption of electricity (ca 35 kWh/kg Li, 11 kWh/kg sodium and 11 kWh/kg magnesium). Apart from the amount of energy required thermodynamically, the overall energy efficiency of monopolar electrolysis cells currently in use is only about 40% to 50%. With the rising cost of electricity and the increasing awareness of the carbon dioxide emissions caused as a result of electricity use, there is ever increasing pressure in the producers of these metals to increase the efficiency of the process used.
In addition, the current electrolysis cells used commercially for alkali metal production use cylindrical anodes which results in a low space time yield per cell of such cells.
A further consequence of using cylindrical anodes is that when the diameter of the anode is increased in order to increase the effective surface area of the anode available for electrolysis, the cross sectional area of the anode increases with the square of the diameter of the anode while the periphery increases only linearly. This is an important consideration since graphite, which is used as anode is a very good conductor of heat but not such a good conductor of electricity. In order to carry the necessary current without causing excessive electrical potential losses, the anodes must have a relatively large cross sectional area. Unfortunately this results in a lot of heat loss through the anode.
It is believed that the closest prior art to produce sodium and/or lithium is the so called Downs cell ( US Patent No. 1,501,756 ) and variants thereof. The cell consists basically of up to 8 cylindrical graphite anodes that protrude through the bottom of the cell body into the cell. Around each anode is a cylindrically shaped diaphragm made of steel mesh, a perforated plate or a slotted steel plate. Then there is a cylindrically shaped steel cathode around each diaphragm. The steel cathodes are normally connected to the power source of the cell through connections protruding through the side walls of the cell.
Above the cathodes, there is an annular metal collector made of steel that has the function to collect the bulk of the molten metal product that is produced at the cathodes and that floats upwards into the collector from where it is taken out of the cell. The diaphragms are normally connected and supported by the metal collector.
Above the anodes, there is a chlorine collector or hood made of a high nickel alloy or of refractory lined steel. All the chlorine produced at the anodes is collected in this hood before it flows out of the cell.
During operation chlorine is produced at the anode causing a reduction in the effective density of the electrolyte at the anode which in turn results in an upward flow of the electrolyte. The cell construction is such that some of the electrolyte may flow through the diaphragm over the cathode into the space below the metal collector.
Whereas the chlorine bubbles disengages rapidly from the electrolyte, most of the chlorine bubbles in the circulating electrolyte is stopped by the diaphragm and do not flow with the electrolyte into the space below the metal collector. However, some still do and this results in a loss in the current efficiency of the cell.
Two specific measures described in prior art to reduce this flow are:

Introducing a hollow cavity in the anode with perforations between the hollow inside of the anode and the cylindrical outer surface of the anode. In this way the flow of electrolyte through the diaphragm into the space below the metal collector is reduced since some of the electrolyte flows downwards in the cavity before circulation back into the space between the anode and diaphragm (See US 3,544,444 and US 3,507, 768 )
Introducing downward sloping slots in the diaphragm away from the anode towards the cathode which helps to prevent chlorine bubbles from flowing through the diaphragm ( US 5,904,821 ).

A second prior art closest to the invention is in the field of magnesium electrowinning i.e. the cell developed by IG Farben Industry in Germany (Magnesium Encyclopedia http://www.magnesium.com/w3/data-bank/article.php?mgw=81&magnesium=67, 29 May 2013) or the diaphragm-type magnesium electrolysis cell (Electrolytic Production of Magnesium, 1972, p251, Translated from Russian by J. Schmorak). The brick-lined cell is divided into four to six compartments by semi-submerged refractory partition walls labelled semi walls. Three to five water- or air-cooled graphite anode plates are installed and tightly sealed in the refractory cover of the cell. The semi walls on each side of the anodes separate the magnesium metal and the chlorine gas. Steel cathode plates are installed through the cell cover or through the sidewalls in the cathode compartments.
The sequence of electrodes in the cells is: cathode, anode, cathode, cathode, anode, cathode, cathode anode etc. The design of the cell is such that the flow of electrolyte caused by the chlorine bubbles produced at the anodes is upwards along the anode face, over the cathode into the space below the magnesium collection zone above the zone between the cathodes, downward behind the cathodes and then finally below the cathodes back into the space between a cathode and anode.
The design may result in the flow of electrolyte from the space between the electrodes over or through the cathode into a metal collection zone. Small chlorine bubbles may be entrained in this flow and end in the metal collection zone of the cell which is undesirable.
A need thus exists for an electrochemical cell for producing metallic alkali metals and alkaline earth metals, such as magnesium, from molten salt by electrolysis which alleviates at least some of the shortcomings of the prior known electrochemical cells.

Summary of the Invention

According to a first aspect of the invention, there is provided an apparatus for the production of metallic alkali metals, M, or alkaline earth metals, M_ae from the molten salts thereof, the apparatus including at least an electrochemical cell with planar anodes and cathodes installed in the following sequence: (a-c-a)_n to produce alkali metal or alkaline earth metals electrolytically from the respective chloride salts according to the following reactions: $2 MCl = 2 M + {Cl}_{2}$
$2 M_{ae} {Cl}_{2} = M_{ae} + {Cl}_{2}$
In the above, (a-c-a)_n represents the arrangement wherein the sequence of anodes and cathodes of anode-cathode-anode are repeated n number of times, as required anode, cathode, anode, anode, cathode, anode, anode, cathode, anode, and so on.
The alkali metal, M, is typically lithium or sodium.
The alkaline earth metal M_ae, is typically magnesium.
Diaphragms which may be made of steel mesh, perforated plate, or slotted plates may be installed between each pair of opposed anodes and cathodes.
A metal collector assembly may be installed above each cathode to collect molten alkali metal or alkaline earth metal that floats to the top of the electrolyte from where it is withdrawn from the cell.
The metal collector assembly may be electrically isolated from both the anodes and cathode of an anode-cathode-anode set while the metal collector assembly and the diaphragms are electrically connected to each other.
Both the metal collector assembly and the diaphragms may be cathodically protected by molten alkali metal or alkaline earth metal collected in the assembly for example, by molten Li, Na and/or Mg.
In use, chlorine gas produced at the anode or anodes may cause circulation of a molten electrolyte used in the cell upwards along the face of the anode surface in the spaces between each anode and opposing cathode, over the active anode body, then downwards behind the anode body before turning around to flow upwards again over the face of the anode (or anodes).
Chlorine produced at the anodes may disengage from the circulating electrolyte at the top of the molten electrolyte above the active electrolysis zones between the anodes and cathodes. The chlorine thus produced in the head space above the electrolyte is withdrawn from the cell and may be used for various purposes.
A particular configuration of the apparatus is to install the cathodes through the bottom of the cell and the anodes through the side or opposing sides of the cell. However, it is also feasible to install the cathodes and anodes through the other faces of the cell, including the top of the cell.
The electrochemical cell may be lined with chlorine resistant refractory or be made of metal, provided that the metal exposed to chlorine gas in the head space of the cell is sufficiently resistant to attack by chlorine, e.g. nickel or a high nickel containing alloy.
Suitable feed means may be installed to feed the salt to be electrolysed into the electrochemical cell and suitable withdrawal means may be provided to withdraw the alkali or alkaline earth metal and chlorine produced in the cell from the cell.
Furthermore suitable heaters may be provided to preheat the electrochemical cell to melt the electrolyte inventory in the cell before commencing electrolysis.
A suitable direct current power source may be supplied to provide the electrical potential and current required for the reaction.
According to a second aspect of the invention, there is provided a process for the production of alkali metals and alkaline earth metals from the molten salts thereof by electrolysis, said method including

arranging three or more electrodes in an (a-c-a)_n arrangement in an electrolysis cell;
maintaining an electrical potential between the electrodes sufficient for the electrolytic decomposition of the alkali or alkaline earth metal salt in the electrolysis cell;
feeding the molten alkali metal or alkaline earth metal salt into the electrolysis cell;
permitting the gas produced at the anode to cause circulation of the molten electrolyte used in the cell upwards along the face of the anode surface in the spaces between each anode and opposing cathode, over the active anode body, then downwards behind the anode body before turning around to flow upwards again over the face of the anode (or anodes); and
permitting the metallic alkali metal or alkaline earth metal to become separated by density from the molten electrolyte and thus recovered.

The process may include maintaining the metallic alkali metal or alkaline earth metal under an inert atmosphere during extraction thereof.
The molten alkali metal or alkaline earth metal salt may be a sodium, lithium or a magnesium salt.
The sodium salt may be NaCl in which case the electrolyte may contain NaCl, CaCl₂, and BaCl₂ allowing the cell to be operated at temperatures from about 550 to 700°C.
The lithium salt may be LiCl in which case the electrolyte may consist predominantly of a mixture of KCl and LiCl allowing the cell to be operated at temperatures from about 400 to 500°C.
The magnesium salt may be MgCl₂ in which case the electrolyte may consist predominantly of a mixture of KCl, NaCl, CaCl₂, BaCl₂ and MgCl₂ allowing the cell to be operated at temperatures from about 660 to 800°C.
The alkali metal or alkaline earth metal may be recovered at a temperature above its melting point, in liquid form.
The alkali metal or alkaline earth metal may be recovered at a temperature below the melting point of the alkali metal or alkaline earth metal salt from which the alkali metal or alkaline earth metal is recovered.
In the process, chlorine produced at the anodes may disengage from the circulating electrolyte at the top of the molten electrolyte above the active electrolysis zones between the anodes and cathodes.
The chlorine thus produced in the head space above the electrolyte may be withdrawn from the cell and may be used for various purposes.

Description of an Embodiment of the Invention

The invention will now be described, by way of two non-limiting examples only, with reference to the accompanying diagrammatic drawings, Figures 1 to 4. In the different figures the same numerals are used to indicate the same components of the illustrated cells. In different variations of the invention, the anodes of the cell may be installed to protrude through the bottom, side or top of the cell whereas the cathodes may be Installed to protrude through the bottom or the side of the cell.
Figure 1 shows a vertical cross sectional schematic through the first example of an operating electrochemical cell of the invention where the anodes protrude through two opposing sides of the cell and the cathodes of the cell protrude through the bottom of the cell. Figure 2 shows a horizontal section schematic viewed from the top of the construction of the electrochemical cell shown in Figure 1.
The cell (1) may have a shell (24) that may be constructed from steel. The cell may have a removable lid (11). The cell may have a refractory lining (10) that serves to protect the shell of the cell against the hot molten electrolyte inside the cell, to limit thermally induced stresses caused by temperature increases of the shell, and to limit heat losses from the cell. Four planar anodes (19) and two planar cathodes (23) of the cell (1) are shown in Figure 1. The anodes and cathodes are arranged in the following order: anode-cathode-anode-anode-cathode-anode, or (a-c-a)₂, Each cathode is separated from the opposing pair of anodes by a diaphragm (20) and a metal collector (18) is positioned above each cathode to collect the molten alkali metal or alkaline earth metal (17) that is produced on the surfaces of the cathode. The molten alkali metal or alkaline earth metal (17) has a lower density than the electrolyte (15) and therefore floats into the metal collector (18). The piping to remove the molten metal from the metal collector is not shown in Figure 1. The alkali metal or alkaline earth metal salt feed (14) to the cell is introduced into a feed vessel (13) where it is dissolved in electrolyte that is circulated from and back to the cell via hot pipe lines (21) between the feed vessel (13) and the cell. The cathodes (23) protrude through the shell (24) and refractory lining (10) of the cell through the bottom of the cell. The mounting (22) of each cathode serves to position the cathode, to insulate it from the shell (24) and also to cool the cathode (23) as dictated by the thermal design requirements of the cell. Details of the mounting are not shown, since means to achieve the mentioned objectives are well-known in the field. Chlorine is produced on the surfaces of the anodes (19) and rises as gas bubbles (16) towards the surface of the electrolyte bath where it disengages from the electrolyte and exits the cell through an exit port (12). Typically the alkaline earth metal is Mg.
The gas bubbles (16) form predominantly on the vertical surfaces of the anodes (19) on the sides opposing the vertical surfaces of the cathodes (23). Whereas virtually no bubbles are formed on the vertical surfaces of the anodes on the opposite sides of the anodes, the bulk density of the electrolyte/bubble mixture in the spaces between the anodes and the diaphragm (20) is lower than the bulk density of the electrolyte in the space (25) between two opposing anodes and also to that of the electrolyte in the space between the anodes and the inner surface of the refractory lining (10). This difference in density causes the electrolyte to flow upwards in the space between the anode (19) surfaces and the diaphragms and downwards in the latter spaces as indicated by the broken arrows. Similarly, because the diaphragm prevents the passage of chlorine bubbles, the density of the electrolyte bubble mixture in the space between the anodes (19) and diaphragms (20) is lower than that of the electrolyte in the space between the diaphragms (20) and the cathodes (23). Some electrolyte therefore flows through each diaphragm in the upper part of the diaphragm towards the cathode, then downwards and lastly back through the diaphragm (20) in the lower part of the diaphragm towards the anode (19) opposite the specific diaphragm (20). Such flow is essential to replenish the alkali metal or alkaline earth metal cations that are reduced to metal on the cathode (23) surfaces but if such flow is too high, chlorine bubbles may pass through the diaphragm (20) and eventually rise into the metal collectors (18) where it reacts undesirably with the collected molten metal. By appropriate design of the diaphragms, the spacing between the different electrodes and diaphragms and the spacing between the electrodes and the inner surface of the cell refractory lining (10), the electrolyte flow around the anodes (19) are significantly increased relative to the circulating electrolyte flow through the diaphragms (20).
In Figure 2 it is shown how four units of anode-cathode-anode assemblies can be installed in a single cell with two assemblies on each side of the cell when the anodes (19) protrude through two opposing side walls of the cell and the cathodes (23) protrude through the bottom of the cell. Also shown are anode mountings (26) that similarly to the cathode mountings (22) shown in Figure 1 serve to position the anodes, to insulate the anodes from the shell (24) and to cool the anodes. Details of the anode mountings (26) are not shown.
Figure 3 and Figure 4 illustrate diagrammatically the design of a second example of a cell designed in accordance with the invention where the anodes protrude through the bottom and the cathodes through a side wall of the cell. Figure 3 shows a vertical cross sectional schematic through an operating electrochemical cell and Figure 4 shows a vertical cross section through the construction and one of the anodes of the cell at a 90° angle relative to the cross section shown in Figure 3.
In order to allow efficient circulation of electrolyte over the anodes, downwards in the spaces between two opposing anodes or the space between anodes and refractory lining (10) of the cell and then back to the space between the anodes and the diaphragms of the cell, slots (27) or other suitable flow channel are provided in the anodes. The circulation is caused by the same density differences as described in the first example.
The installation of the anodes (19) through the bottom of the cell and the cathodes (23) through a side wall of the cell is illustrated in Figure 4. The slots (27) through the anodes are also shown and a diaphragm (20) behind the anode, A metal collector (18) is positioned on top of the cathode (23) and the shown diaphragm (20) and it may typically be sloped to direct the flow of molten metal towards the top of the collector from where it is withdrawn through pipe work that is not shown.
In the invention as illustrated in the Figures, in contrast to current planar magnesium electrolysis cells that are designed to induce large molten electrolyte circulation flow currents of the electrolyte rising at the anode surface to the metal collectors above the cathode, the new design minimizes mixing caused by such flow patterns.
Furthermore, whereas the circulating electrolyte contains both dissolved chlorine and small chlorine bubbles, circulation of the electrolyte from the anode surface to the metal collection zone in the current planar electrode arrangements causes mixing of and back reaction of such chlorine with the molten metal in the metal collection zone.
Apart from a reduction in efficiency by this, back reaction of Cl₂ with Li, Na or Mg causes the formation of LiCl, NaCl or MgCl₂ in the metal collection zone, LiCl and NaCl in particular have melting points that are substantially higher than the melting points of the electrolytes used with the result that solid salt is deposited in the metal collection zone that can cause blockages of the molten metal withdrawal lines. MgCl₂ also has a higher melting point than the electrolyte, but the difference is substantially less. Many Mg cells actually operate above the melting point of MgCl₂ that is ca 714°C.
In the apparatus of the present invention as illustrated in the Figures, the electrochemical cell design of the invention causes a large molten electrolyte flow pattern upwards along the face of the anode surface, over the active anode body, then downwards behind the anode body before turning around to flow upwards again over the face of the anode (or anodes),
Thus, in the cell of the invention, little mixing occurs between the circulating electrolyte and the molten metal collected in the metal collector resulting in an increase of the overall current efficiency.
In contrast to the designs using cylindrical anodes, planar anodes and cathodes are used which enhances up-scaling of the cell and also increases the packing density of electrodes in the cell.
It is believed that the invention as illustrated has certain advantages over the known electrochemical cell configurations in that:

Reduced electricity consumption per unit of metal produced resulting in lower operating costs
Higher space time yields leading to smaller cells and less electrolyte inventory both resulting in lower capital costs.
Simpler cell design to scale-up resulting in lower capital costs for a metal production plant.
An extra degree of freedom to optimize the design when using planar rather than cylindrical anodes.
A simpler design and hence lower cost of the chlorine and metal collectors above the electrodes relative to that installed in typical Downs type cells

Claims

An apparatus for the production of a metal selected from metallic alkali metals, M, and alkaline earth metals, M_ae from the molten salts thereof, the apparatus characterised by including at least an electrochemical cell with planar anodes and cathodes installed in the following sequence: (a-c-a)n to produce alkali metal or alkaline earth metals electrolytically from the respective chloride salts thereof, wherein n represents the number of times the sequence of anode-cathode-anode is repeated, and wherein a metal collector assembly is installed above each cathode to collect molten alkali metal or alkaline earth metal that floats to the top of the electrolyte from where it is withdrawn from the cell.
An apparatus as claimed in claim 1, wherein the alkali metal, M is selected from lithium and sodium and the alkaline earth metal M_ae is magnesium.
An apparatus as claimed in claim 1 or 2 wherein diaphragms which are made of steel mesh, perforated plate, or slotted plates are installed between each pair of opposed anodes and cathodes.
An apparatus as claimed in claim 1, wherein the metal collector assembly is electrically isolated from both the anodes and cathode of an anode-cathode-anode set while the metal collector assembly and the diaphragms are electrically connected to each other.
An apparatus as claimed in claim 3 or 4, wherein both the metal collector assembly and the diaphragms are cathodically protected by molten alkali metal or alkaline earth metal collected in the assembly.
An apparatus as claimed in any one of the preceding claims, wherein the electrochemical cell is lined with chlorine resistant refractory material or is made of metal, provided that the metal exposed to chlorine gas in the head space of the cell is sufficiently resistant to attack by chlorine.
An apparatus as claimed in any one of the preceding claims, wherein a direct current power source is supplied to provide the electrical potential and current required for the reaction.
A process for the production of metal selected from alkali metals and alkaline earth metals from the molten chloride salts thereof by electrolysis, said process characterised by
- arranging three or more electrodes in an (a-c-a)n arrangement in an electrolysis cell, wherein n represents the number of repetitions of the anode-cathod-anode electrode arrangement;

- maintaining an electrical potential between the electrodes sufficient for the electrolytic decomposition of the alkali or alkaline earth metal salt in the electrolysis cell;

- feeding the molten alkali metal or alkaline earth metal salt into the electrolysis cell;

- permitting the gas produced at the anode to cause circulation of the molten electrolyte used in the cell upwards along the face of the anode surface in the spaces between each anode and opposing cathode, over the active anode body, then downwards behind the anode body before turning around to flow upwards again over the face of the anode (or anodes);

- permitting the metallic alkali metal or alkaline earth metal to become separated by density from the molten electrolyte and thus recovered; and

- maintaining the metallic alkali metal or alkaline earth metal under an inert atmosphere during extraction thereof.
A process as claimed in claim 8, wherein the molten alkali metal or alkaline earth metal salt is selected from sodium, lithium, or a magnesium salt.
A process as claimed in claim 9, wherein the sodium salt is NaCl in which case the electrolyte contains NaCl, CaCl₂, and BaCl₂ allowing the cell to be operated at temperatures from about 550 to 700°C.
A process as claimed in claim 9, wherein the lithium salt is LiCl in which case the electrolyte consists predominantly of a mixture of KCl and LiCl allowing the cell to be operated at temperatures from about 400 to 500°C.
A process as claimed in claim 9, wherein the magnesium salt is MgCl₂ in which case the electrolyte consists predominantly of a mixture of KCI, NaCl, CaCl₂, BaCl₂, and MgCl₂ allowing the cell to be operated at temperatures from about 660 to 800°C.
A process as claimed in any one of claims 8 to 12, wherein the alkali metal or alkaline earth metal is recovered at a temperature above its melting point, in liquid form.
A process as claimed in any one of claims 8 to 12, wherein the alkali metal or alkaline earth metal is recovered at a temperature below the melting point of the alkali metal or alkaline earth metal salt from which the alkali metal or alkaline earth metal is recovered.
A process as claimed in any one of claims 8 to 14, in which chlorine produced at the anodes disengages from the circulating electrolyte at the top of the molten electrolyte above the active electrolysis zones between the anodes and cathodes.