WO1997028295A1 - Method and apparatus for electrolysing light metals - Google Patents

Abstract

The present invention provides a new and useful process for the production of a molten metal by electrolysis in an electrolytic cell having an electrolysis section (16), a metal recovery section (18) and a submerged reservoir (68), said process comprising electrolysing in said electrolysis section of said cell an electrolyte containing a fused salt of said metal to produce said metal, said electrolyte having a greater density than said metal; causing said metal and additional said electrolyte to circulate continuously from said electrolysis section to said recovery section; continuously separating said metal from said electrolyte in said recovery section; causing said metal to circulate toward a part of said recovery section remote from said electrolysis section; conveying said metal from said recovery section to said submerged reservoir; and periodically recovering said metal from said reservoir.

Description

TITLE OF THE INVENTION

METHOD AND APPARATUS FOR ELECTROLYSING LIGHT METALS

FIELD OF THE INVENTION

This invention relates to improved processes and apparatus for the production of molten metals by electrolysis of their fused salts where the metal is lighter than the electrolyte. More particularly, the invention relates to improved method and apparatus to collect molten metals such as lithium, magnesium, or sodium in electrolytic cells of monopolar and multipolar design.

BACKGROUND OF THE INVENTION

All electrolytic cells that are used to commercially produce lithium, magnesium or sodium utilize an electrolysis section above which the electrolysis gas is collected, and a metal recovery section in which the metal collects and is stored between tappings. Between the two sections is a partition. As a common feature, this partition is usually immersed deep in the electrolyte to effect good separation of the electrolysis gas and long storage of the metal produced. This partition, sometimes called a curtain wall or semi-wall, is a critical component of the cell due to the reactivity of the gas and/or the metal and the consequent need to maintain their separation, but the partition is usually one of the components that limit the operating life of a cell due to wear and cracking. The chemical wear of the curtain wall in contact with the metal may be responsible for some loss of product metal purity, and cracks in the curtain wall result in leaks of metal and air into the electrolysis section with consequent oxidation of the graphite anodes and back reaction of the metal with the electrolysis gas. PRIOR ART

U.S. patent 1 ,501 ,756, issued 15 July 1924 to Downs, describes a process commercially used to produce sodium from sodium chloride. The process uses for the collection of the molten sodium an upper reservoir which is separate from the electrolysis cell itself.

U.S. patent No. 3,396,094, issued 6 August 1968 to Sivilotti et al., describes an electrolytic magnesium cell that is provided with a metal collecting reservoir, located in the metal section and almost wholly submersed in the electrolyte. The reservoir consists of an inverted box of steel along the partition above openings through the curtain wall. The reservoir is open along its bottom to receive the metal that comes through the openings through the curtain wall. This metal collection arrangement was superior to the prior art, where the metal was allowed to float freely on the surface of the electrolyte. It allowed the cell to operate with the electrolyte temperature near the melting point of the metal, which resulted in substantial improvement of the current efficiency of the cell. The metal had to be maintained molten to be tapped out of the cell by conventional siphon means, and the fact that the metal was maintained under the surface of the electrolyte equalized the two temperatures without need of supplementary heating means. Relatively large quantities of metal were collected and the need for undue frequency of tapping was avoided.

It was subsequently found that oxidation of the residual floating metal that escaped collection into the reservoir and hydrolysis of the electrolyte were detrimental to the operation of the cell. Sludge formation, short cell life and upsets in current efficiency were still experienced.

A fully enclosed cell provided with an insulating cover, with an inert gas blanket and with internal temperature control means, was developed as described in U.S. patent 4,420,381. The heat exchanger had to be well insulated where it passed through the floating metal pad in order to avoid premature freezing of the metal. The design of U.S. patent 4,420,381 was an improvement over the previous art and has been used with other more recent improvements in cell design. These improvements are related to the use of new electrode geometries, in particular those of multipolar design, that substantially increase cell productivity and decrease unit energy consumption. These improved cells are described in U.S. patents Nos. 4,055,474; 4,514,269; 4,518,745; 4,604,177 and 4,960,501 , which are incorporated herein by reference. These cells require an even tighter control of the temperature and of the oxidation reactions. Also, they are producing at a high rate so that the volume of metal to be stored in the metal section between tappings is very large. Additionally, for good current efficiency, the multipolar cells require an almost constant level in the electrolysis section. This can be obtained by feeding the cells continuously in response to level sensing means, or by regulating the supply of inert gas to and from a submersed open-bottom reservoir, to compensate for liquid volume changes when feeding and tapping are carried out intermittently.

In the cell described in U.S. patent 4,518,745 the electrolyte circulation towards the metal section occurs sideways in the planes of the inter-electrode spaces and over a weir, located inside the electrolysis section, downstream from the electrodes and upstream from the curtain wall. The electrolyte/metal mixture flows over the weir so that the level above the electrodes remains almost constant. However, the turbulence downstream from the weir entrains residual gas within the electrolyte flowing into the metal section. Also, the turbulence hinders coalescence of the metal that would help its rising towards the floating metal pad.

Coalescence could be a significant factor to improve the current efficiency of multipolar cells, as it is believed that droplets which are smaller than a critical size and are recirculated in the electrolysis section are consumed by back reactions in the inter-electrode spaces (see Sivilotti O.G., Operating Performance of the A/can Multipolar Magnesium Cell, Light Metals, 1 17th AIME Annual Meeting, Phoenix, 1988). The critical size of the metal droplets depends on the degree of turbulence and on the path of the circulating electrolyte. Therefore, the geometry of the metal section where the metal separates by upwards settling is very important to obtain high current efficiency.

U.S. patent No. 5,417,815, issued 23 May 1995 to Robinson et al., describes the prior art for apparatus and methods to produce lithium metal from molten mixtures of lithium chloride and other metal chlorides. The patent describes a liquid metal skimmer based on the use of mechanical propellers in a draft tube. Devices based on mechanical moving parts are difficult to maintain in continuous reliable operation because of the high- temperature molten-salt environment.

While satisfactory operation has been obtained with cells of the prior art, the present invention is designed to obtain significant improvements in such cells and in their method of operation. The main objectives are a better current efficiency and improved yield and recovery of purer metals, as well as greater convenience in the collection and removal of the metal. Cheaper construction and longer operating life result in lower capital costs and lower maintenance expenses.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process to electrolytically produce at high current efficiency lithium, magnesium, sodium and other molten metal products that are lighter than the electrolyte.

Another object of the invention is to provide a process to electrolytically produce reactive light metals of high purity.

A further object of the invention is to provide a method for efficiently separating a light metal from an electrolyte stream and for facilitating its tapping at infrequent intervals.

A still further objective of the invention is to provide an electrolytic cell of long life and of cost effective construction for the production of metals lighter than the electrolyte.

Thus in one broad embodiment the invention provides a process for the production of a molten metal by electrolysis in an electrolytic cell comprising a process for the production of a molten metal by electrolysis in an electrolytic cell having an electrolysis section, a metal recovery section and a submerged reservoir, said process comprising electrolysing in said electrolysis section of said cell an electrolyte containing a fused salt of said metal to produce said metal, said electrolyte having a greater density than said metal, causing said metal and additional said electrolyte to circulate continuously from said electrolysis section to said recovery section, continuously separating said metal from said electrolyte in said recovery section, causing said metal to circulate toward a part of said recovery section remote from said electrolysis section, conveying said metal from said recovery section to said submerged reservoir, and periodically recovering said metal from said reservoir.

In a further broad embodiment the invention provides an electrolytic cell comprising an electrolysis section, a metal recovery section continuous with said electrolysis section, a submerged reservoir for storing a product metal, and means for conveying a product of electrolysis from said metal recovery section to said reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the invention will become apparent upon reading the following detailed description and upon referring to the drawings in which:-

Figure 1 is a vertical cross-section front to back through a cell according to the invention;

Figure 2 is a plan view partly in section of the cell of Figure 1 ; Figure 3 is a vertical transverse cross-section of the cell of Figure 1 ;

Figure 4 is a plan view partly in section of another embodiment of the cell of Figure 1 ;

Figure 5 is a schematic cross-section of a transfer pump in position for use in a cell according to the invention;

Figure 6 is a schematic cross-section through a part of a cell and a syphon arrangement for use with the cells of the invention;

Figure 7 is a schematic vertical cross-sectional view of an apparatus according to the invention;

Figure 8 is a schematic vertical cross-sectional view of the apparatus of Figure 7;

Figure 9 is a schematic horizontal cross-sectional view of the apparatus of Figure 7;

Figure 10 is a schematic view of a heat exchanger for use in the electrolytic cell of Figures 7 to 9; and

Figure 1 1 is a vertical cross-section illustrating a further embodiment of the invention.

While the invention will be described in conjunction with the illustrated embodiments, it will be understood that it is not intended to limit the invention to such embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

As is evident in the prior art, and in any event to those skilled in the art, the invention is in the context of electrolytic cells which are divided into electrolysis and metal recovery sections which have conventionally been separated by a partition or curtain wall. When the cell is in operation a natural circulation is set up brought about by the liberation of gas in inter-electrode spaces. As the gas rises, it functions as a pump to set up circulation within the cell. Various means have been used to direct the circulating stream along the upper part of the cell from the electrolysis section to a metal recovery section and hence downward to the lower part of the metal recovery section and back to the lower part of the electrolysis section under the electrodes. In the metal recovery section a floating metal pad is formed and is tapped, generally on an intermittent basis. At an appropriate point in the cycle the cell is fed to enrich the electrolyte.

Two general criteria are required to obtain current efficiencies that are as high as, or close to, those obtainable in electrolytic cells that collect the metal at the cathode and keep it separate from the electrolysis gas (as for example U.S. patent 3,396,094). First, the metal droplets that are released in the inter-electrode space and are entrained in the circulating electrolyte must spend the shortest possible time in the inter-electrode space; and, second, the droplets must separate from the electrolyte into a metal pad regardless of their small size. To meet the first criterion the electrolyte is made to circulate as fast as possible in the inter-electrode space and to meet the second criterion, notwithstanding the fast electrolyte flow, means are provided to obtain coalescence and separation of metal droplets before the electrolyte is returned to the bottom of the inter-electrode space.

Contrary to earlier belief, it has now been discovered that coalesced metal droplets (and even a small metal pad) floating on the surface of the electrolysis section do not contribute significantly to loss of current efficiency, as a film of electrolyte coats the surface of the metal and prevents the direct contact between the metal and the electrolysis gas, when good wetting conditions between metal and electrolyte are maintained. Thus a partition or other structural barrier may not always be necessary to maintain separation between the gas and metal.

To meet other objectives of the invention, the separated metal must be maintained out of contact with the refractory walls as much as possible to prevent reaction with the latter and consequent contamination of the metal. This is to be obtained notwithstanding the desirability, for efficient operation, of tapping as infrequently as possible the metal produced.

The fact that the reaction between the refractory walls and the metal is prevented and the fact that the cell is sealed to eliminate metal oxidation and electrolyte hydrolysis are further requirements to obtain high current efficiency, high yields and long operating life.

In reference to Figures 1 to 3, the apparatus illustrated is an electrolytic cell 10 having a structural steel casing 12 lined with a layer of insulating and refractory material 14 suitable to contain a molten salt electrolyte. The cell 10 is divided into an electrolysis section 16, a metal recovery section 18, and a services section 20, the last separated from the other sections by a semi-wall, partition, or curtain wall 22.

Cell 10 comprises back wail 24, front wall 26 and side walls 28 and 30. In one preferred configuration the partition wall 22 extends diagonally across the front corner of cell 10 from front wall 26 to side wall 28 (or 30).

In a further and most preferred configuration the services section 20 is external to ceil 10 and is defined by a set of side walls 32, 34 and 36. In this configuration the partition wall 22 comprises a part of side wall 28 of cell 10.

The cell 10 is provided with top 38 which may be in sections for convenience of handling and which seals the cell, including the services section 20, when the cell is in operation. The partition wall 22 is preferably integral with a section of top 38 and extends downwardly a short distance below surface 40 of electrolyte 42 to thereby seal services section 20 against entry into that section of electrolysis gas liberated into space 44 between surface 40 of electrolyte 42 and top 38 of cell 10.

Below surface 40 of electrolyte 42 and below the bottom 46 of partition wall 22, the services section 20 is open to electrolyte 42 in cell 10.

The electrolysis gas disengages from the electrolyte at top 48 of the electrodes 50 and is collected under the refractory-lined cover 38. The gas is withdrawn under slightly negative pressure through a gas duct schematically shown by the arrow 51.

The arrow 52 indicates the location of feed entry into the cell 10 through refractory-lined lid 38 when the cell is to receive solid feed.

In the preferred arrangement, the anodes 54 and cathodes 56 are disposed along the back wall 24 of cell 10 and provide facing surfaces for the electrolysis process. One or more bipolar electrodes 57 is (are) interposed between anode and cathode when a multipolar structure is used. The gas generated on the anodic surfaces provides the pumping action to the electrolyte as the gas rises in the inter-electrode spaces. The electrolyte carries entrained metal droplets with it.

As shown in the horizontal views of Figure 2 and 4, the anodes are preferably wedge shaped, with decreasing cross-section outwardly from back wall 24, thus pointing toward the front of the cell, while the cathodes are opposite. The anodes are preferably though not necessarily pointed.

This geometry is more advantageous when the anode leads 58 are mounted through the back wall 24 of cell 10, as the current flows in the body of the anode at uniform current density from the root of the anode to the pointed end 60. The cathode leads 62 are also mounted through the back wall 24 of cell 10, preferably through the bottom part of wall 24, in order to reduce the danger of short-circuits through the electrolyte-wetted refractory lining. The lining may be rapidly destroyed by such event. Alternatively, to further reduce this danger, the cathode leads may be mounted through the bottom of the cell 10, but the connection to the cathode busbar will be more difficult. For electrical insulation reasons, both the anode and the cathode leads 58 and 62 are isolated from the cell casing. As an additional precaution, part 63 of the casing 12 that surrounds the anode leads 58 is electrically insulated from the rest of casing 12 by spacers 64.

The electrolyte/metal mixture flows along the cathode 56 toward the front of cell 10. The wedge-like geometry of the cathode is particularly useful in providing to the electrolyte a linearly increasing cross- sectional area that matches the increasing volume of the electrolyte discharged along the top of the cathode. In this way, after the discharge, the turbulence is minimized and the metal droplets entrained in the electrolyte start to coalesce immediately. The space between the non- working faces of the cathode may be filled with a set of metallic nets or other conventional means to help the metal coalescence.

Contrary to earlier cells, it must be understood that the electrolyte flow velocity is slowed while still in the electrolysis section, by reason of cell geometry, so coalescence, as indicated, begins in the electrolysis section.

An important aspect of the invention is the continuity between the metal recovery section 18 and the electrolysis section 1 6, the former extending to the front wall 36 of cell 10. Leaving the electrolysis section and in the metal recovery section 18, the electrolyte flows at very low velocity, so that the time for the metal droplets to separate from the electrolyte is maximized.

A bottom wall 66 of metal recovery section 18 forms the sloping roofs of two open-bottom reservoirs 68 and 70, which are set in cascading and sealing sequence along the return flow path of the electrolyte in the lower part 67 of cell 10. Reservoir 68 provides storage capacity for the metal produced between tapping cycles; and reservoir 70, for the inert gas required to compensate for volume changes during intermittent tapping and feeding operations.

Reservoir 68 is an open-bottom steel box that runs along the front wall 26 preferably in sealing abutment with the side walls 28 and 30 and front wall 26. It is supported on a ledge 74 along wall 26 and on similar ledges on the side walls 28 and 30 of the cell. Reservoir 70 is similarly supported in sealing abutment on the front end of the cathodes at 76 and on ledges on the side walls 28 and 30. Reservoir 68 is sufficiently heavy to stand firm on its supports, while reservoir 70 should have adequate ballast to keep it in place when full of inert gas.

Reservoirs 68 and 70, as well as heat exchanger 78, are located below the curtain wall 22 and preferably extend into services section 20. When those components must be removed for maintenance, curtain wall 22 must also be removed. Therefore curtain wall 22 is preferably attached to the front section 80 of the cell lid, as noted above. To minimize this problem, more efficient heat exchangers can be used, based on thermosyphon and/or heat pipe designs of simple vertical or gently curved pipe geometry which can be extracted from cell 10 through services section 20 without removal of cell lid 80 or curtain wall 22.

Figure 4 shows a transverse curtain wall, while Figure 2 shows a diagonal geometry. The transverse curtain wall is preferred as the width is smaller and can be built of a single refractory block that can be handled independently. However, in either case, the front wall 26 of the cell 10 remains straight, and, preferably, a series of cells are operated from a platform running along the front of the cells.

The means for conveying metal into reservoir 68 preferably has entry funnel 82 at surface 40 of the electrolyte 42 in the metal recovery section, where the metal collects naturally. There are two preferred means of transfer: an active pump 84 or a skimmer-tube 86. A selection is made depending on the relative density of the metal and the pressure head available. A large hydraulic head is required to force a light metal down into a skimmer-tube, and, therefore, in lithium cells of monopolar design, it is best to use a transfer pump. The opposite is true for magnesium cells of multipolar design where the gas-lift action is strong and the relative metal density is only about 10%. In this case the electrolyte flow through the skimmer-tube could be only a fraction of the total flow, and leaks around the reservoirs can be tolerated. Mathematical and/or physical modelling techniques are used to design the skimmer-tube 86. A good reference is a paper by R. Sankaranarayanan and R.I.L. Guthrie entitled: Vortex Suppression Device Improves Steel Cleanness, 1995 - 14th PTD Conference Proceedings of the Iron and Steel Society. A vortex phenomenon (that is stated to enhance entrapment of the floating slag) may be encouraged in the present invention by locating skimmer-tube 86 away from the centre of symmetry of the cell. The level of electrolyte over the entry funnel 82 and the hydraulic pressure drop through the tube itself is controlled by using level sensing means 88 and feeding or bleeding inert gas into and out of reservoir 70. Level fluctuations of the order of about one centimetre are acceptable for satisfactory performance.

Where a pump is required or desired, conventional rotary pumps may be used. However, a transfer pump design that meets the tough environmental conditions of a fused salt electrolytic cell is described schematically in Figure 5. The body of the pump is a vertical tube 90 partially immersed in the electrolyte and located in the services section 20 out of contact with the electrolysis gas. The bottom of the tube is connected via a non-return valve 92 to the entry funnel 82 and to a bottom discharge nozzle 94, via another non-return valve 96. The non-return valves cause the flow to occur only in the direction from the entry funnel 82 and to the bottom nozzle 94 respectively. The top 98 of the tube 90 is connected to an inert gas supply via a pressure reducer 100 and a non¬ return valve 102. Between the non-return valve 102 and tube 90, a pneumatic accumulator 104 is connected to the inert gas line 106. The bladder 108 of accumulator 104 expands or contracts, depending on whether compressed air is fed into or bled out of the accumulator via three- way valve 109. By periodically switching the three-way valve with solenoids, inert gas is caused to be moved, in known volumes, in and out of the tube 90, causing intermittent flow of liquid in alternating directions through its bottom connection. Thereby the volume is known of fluid transferred from the surface of the electrolyte in the metal recovery section to the region below the metal collecting reservoir 68. By selecting a frequency of operation that matches the volume of metal production, the size of the metal pad that forms at the entry funnel between pump cycles is maintained at an acceptable level. Preferably, the rate of pumping is maintained higher than the rate of metal production and the fluid flow in the transfer pump is a mixture of metal and electrolyte, with the latter making up for the differences.

As well, a parallel path is provided for electrolyte circulation, and this may follow several paths. For example, openings may be provided in the bottom wall 66 of metal recovery section 20, circulation may occur through section 20 under wall 22, etc.

The transfer pump 84 is mounted on the refractory and insulating lid 38 of the services section 20 in such a way that it can be installed and removed for maintenance reasons without removal of the lid 38 or of the curtain wall 22. All the equipment on lid 38 is installed by means of gas-tight flanges so that during operation a slight positive pressure of inert gas can be maintained in space 45.

In order to access cell 10 without exposure to the electrolysis gas in space 44, various entry points are provided into services section 20. Thus, temperature and level sensing means 1 10 and 88, and heat exchanger inlet and outlet 114 and 1 16 are preferably located in services section 20. A tapping spout 1 18 is also located in services section 20 and extends into reservoir 68 to provide access to the reservoir for tapping the product metal. Where a transfer pump is utilized, as discussed above, as a means of conveying product from the metal recovery section to reservoir 68, the pump is also preferably located in services section 20.

In commercial operation, cells of the present invention will be used as part of a bank of multiple such cells. The molten metal can be tapped from reservoir 68 by conventional means, such as syphons attached to vacuum ladles moved to and from the cell by truck on the operating platform conventionally present on the front of the bank of cells. Alternatively, the ladles may be moved by mobile overhead crane.

However, it has been found very advantageous to provide each cell 10 in a bank of cells with metal tapping means connected directly to a hot metal piping system leading from the cells to the cast-house. Preferably a pipeline 120 is located along the front of a series of cells below the operating platform. Pipeline 120 is preferably thermally insulated and is made up of thermostatically controlled modules in a closed loop network in such a way as to secure continuous operation of the cells even when a pipe module must be isolated from the pipeline loop and removed for maintenance.

In order to avoid short-circuiting between cells, the tapping must be performed on a cell by cell basis. When a cell is discharging metal into the pipeline 120 during tapping, a direct electrical connection is set up by the molten metal between the cell and the pipeline so that the pipeline rises to the potential of the cell being tapped, while the rest of the cells are electrically insulated from the pipeline.

The tapping means in each cell 10 preferably consists of a syphon pipe 122 with a leg 124 immersed in the tapping spout 1 18 just below the level of the electrolyte. A second leg 126 is immersed in a downstream trap 128, the liquid level in which is just above the level of electrolyte in cell 10. The lower metal density causes the metal level in the tapping spout 1 18 to be higher than the level in the trap and thus enables the syphon, when primed, to discharge metal from cell 10 to pipeline 120.

Preferably, when the syphon is not in use, it is connected to an inert gas supply which maintains a slight positive pressure in the syphon to avoid ingress of air.

Similarly, electrically and thermally insulating lids 130, 132 and 134 are provided to seal the top of the tapping spout 118, the trap 128 and the entry 136 to the pipeline 120. The spaces below the lids are at all times supplied with inert gas at slightly positive pressure to avoid oxidation of the metal.

To initiate a tapping procedure, the application of vacuum at the top of the syphon causes the metal to move up leg 124 of syphon 122 to the top of the leg 124 and hence into leg 126 to initiate flow. The level in the downstream trap in the syphon is located just above the electrolyte level, so that the flow is maintained through the syphon only if there is metal in the submerged reservoir 68. When the reservoir is empty of metal, the flow will naturally stop, even if the syphon is still primed by the vacuum line. This system preferably includes a pre-set time of operation of the syphon, after which the vacuum line is switched off and the inert gas line activated.

In good operational practice the syphon is preferably pre-heated to operating temperature, prior to initiating the tapping sequence.

With reference to the embodiments of Figures 7 to 1 1 , a process and apparatus are shown in which the partition wall 22 is moved to a position between the electrolysis section and the metal recovery section, thus incorporating the services section into the metal recovery section, while still maintaining both the gas seal for services piping and the desired flows between the electrolysis section and the metal recovery section.

The apparatus illustrated in these figures is an electrolytic cell 210 having a structural steel casing 212 lined with a layer of insulating and refractory material 214, suitable to contain a molten salt electrolyte. The cell 210 is divided into an electrolysis section 216 and a metal recovery section 218, separated by a semi-wall, partition, or curtain wall 220.

In a preferred configuration and for reasons to be discussed below, the cell includes a second partition wall 222 adjacent to partition wall 220 but separated therefrom by the space 224. A conduit 225 leads from space 224 through the top wall of cell 210.

In conventional arrangement, electrolysis section 216 is located adjacent back wall 226 and metal recovery section 218 is located adjacent front wall 228 of cell 210. With reference to electrolysis section 216, electrical leads 230 pass through back wall 226 and are connected to cathodes 234. Cathodes 234 are normally steel.

Similarly, electrical leads 236 pass through top wall 238 of electrolysis section 216 and are connected to anodes 242. Anodes 242 are preferably of graphite.

In a modern multipolar cell, bipolar electrodes 244 are located between anodes 242 and cathodes 234 and each bipolar electrode 244 acts as cathode on one face and anode on the other face, so that the electrolysis process is multiplied by the number of inter-electrode spaces operated within one cell.

With reference to the metal recovery section 218, that section preferably contains an open-bottom reservoir 246 which is partially open at gate 248 to electrolyte 250. Reservoir 246 includes inlet/outlet 252 for injection or removal of inert gas.

As is discussed later, it is essential to control the level of electrolyte 250 within cell 210. That level can be controlled by injecting gas through inlet/outlet 252 to force liquid from reservoir 246 to raise the level of electrolyte 250 within the cell 210 or gas can be withdrawn from reservoir 246 to permit the flow of electrolyte 250 into reservoir 246 to thereby lower the level of electrolyte 250 in cell 210. The use of an open bottom reservoir for purposes of level control is conventional in the art.

Metal recovery section 218 also includes a submerged metal recovery reservoir 254. Reservoir 254 is provided with an entry weir 256, the function of which will be described below, and a port 258 through top wall 260 of metal recovery section 218 through which molten metal 262 in reservoir 254 can be tapped.

Metal recovery reservoir 254 is separated from front wall 228 by space 264. Also located within metal recovery section 218 are at least two horizontal baffles 266 and 268. In the most preferred configuration, and, as illustrated, the baffles are comprised of the tubes 270 of a heat exchanger 272.

As illustrated in Figure 10, heat exchanger 272 comprises inlet 274, outlet 276 and manifolds 278, together with the aforementioned tubes 270.

Reverting back to Figures 7 to 9, a horizontal partition 280 extends between cathodes 234 and a wall 282 of reservoir 254 to form a trough 284.

A second horizontal partition 286 extends from cathodes 234 to a position adjacent tubes 270. Adjacent the baffle 266 a short refractory partition 288 extends from front wall 228 to a position adjacent baffle 266. Baffle 268 is separated from front wall 228 by space 290.

The various structural members in the two sections define flow paths which will be discussed below.

In operation of the cell, electrolysis gas, usually chlorine, is generated on the anodic faces and metal is deposited in liquid form on the cathodic faces. Electrolysis gas in the inter-electrode spaces lifts the electrolyte toward the top of electrolysis section 216, where the gas/liquid phases separate. The gas passes into the space 292 at the top of electrolysis section 216 and is removed therefrom. The gas is preferably removed under a slightly negative pressure to prevent escape.

The upward movement of the electrolysis gas in the inter- electrode space drives the circulation of the electrolyte through the cell.

Driven by the rising gas, the electrolyte 250 circulates toward the top zone 306 of the metal section 218, passing under the semi- wall 220, which is usually, as noted above, of refractory construction to resist the corrosive action of the electrolysis gas. The semi-wall is preferably built as a tight sequence of firebrick blocks anchored to the steel shell 307 of cover 309 of the cell 210, but could also be supported from the floor by refractory or steel piers (not illustrated) or be an arch structure (not illustrated) supported by the sidewalls of cell 210. The refractory material may preferably be made of acid resistant firebrick or of fused alumina or of glass- ceramic material such as PYROCERAM 9606 ™ cordierite as described in U.S. patent 5,429,722.

One important aspect of this embodiment of the invention is that the semi-wall 220 does not go deeply into the electrolyte 250, not to unduly impede the circulation of the electrolyte towards the top of the metal section, as occurs in the cells of the prior art. One important function of the separating wall in the prior art was to contain the metal accumulating in the metal section between tappings. A deep metal pad floating on electrolyte 250 was required also to facilitate tapping by siphon. However, when the metal pad is deep, the liquid metal being lighter than the electrolyte has the tendency to pass through cracks or holes or open joints in the separating wall, and to return to the electrolysis section where it back-reacts with the electrolysis gas.

In the present case, as discussed below, a deep metal pad is not formed, and so the only function of the semi- wall of the present invention is to seal the gas zone 306 at the top of the electrolysis section 216 where the gas readily separates from the electrolyte due to its much lower density.

To prevent carry-over of the residual gas to the metal section by the circulating electrolyte, the second semi-wall 222 can be provided, following the semi-wall 220. A slightly positive pressure of inert gas is maintained in the metal section. The residual gas is released against the bottom 221 of semi- wall 220 and makes its way into the space 224 between the two semi-walls. This gas is then vented out together with some inert gas that leaks through the semi-wall 222 (because of cracks or of its natural porosity), making the metal section 218 free of electrolysis gas. The second semi-wall 222 can be of the same refractory material, as shown in Figure 7, or of metallic material, depending on the corrosive conditions and the quantity of residual electrolysis gas prevailing in the space between the semi-walls. Vent conduit 225, if desired, is connected to gas scrubbing apparatus (not shown) designed to absorb the residual eiectrolysis gas before release to the atmosphere.

To further prevent the carry-over of residual electrolysis gas into the metal section by the circulating electrolyte, the liquid velocity under the semi-wall is reduced by the deep trough 284 located along the semi-walls, into which the semi-walls themselves are slightly immersed. With suspended semi-walls that are not supported from the bottom, the velocity of the electrolyte is minimized relative to the velocity in the turbulent zone 308 above the electrodes, as the total width of the cell is made available to the electrolyte flow, and therefore the electrolyte velocity and the gas carry¬ over is reduced to a minimum. The carry-over of metal droplets, however, is still active, because the small density difference between metal and electrolyte favours the entrainment, and because the metal droplets are still very small as the metal did not have a chance to fully coalesce in the turbulent zone 308 above the electrodes.

The design of the trough 284 for optimum performance of the gas/electrolyte separation and of the electrolyte/metal carry-over functions, could be carried out following the research techniques described in the Al ME paper referred to above. The electrolyte flow path that is made possible by the novel geometry of the semi-walls 220/222 and of the trough 284 of this invention affords a reduced turbulence of the electrolyte in this critical zone under the semi-wall and, therefore, an early onset of coalescence of the metal droplets. The other advantageous feature afforded by this aspect of the invention is the fact that the streamlined flow of the electrolyte reaches the very surface of the metal section where metal separation coalescence naturally occurs resulting in increased metal collection efficiency.

In the preferred case, for the sake of more reliable sealing and better stability of liquid level 310 in the electrolysis section 21 6 above the electrodes 234/244, is located an overflow weir 312 downstream from the trough 284. The geometry is chosen to reduce the turbulence due to this weir to a minimum. The trade-off is between the advantage of increased reliability of the gas seal and increased control of the bypass current on top of the bipolar electrodes; and the loss of some metal coalescence and separation because of the increased electrolyte velocity over the weir, but this loss can be minimized by round-shaping the cross section of the weir itself, as it is conventionally practiced in overflow weirs used in spillways and in other large-scale water works.

As illustrated in Figures 7 and 8, weir 312 is conveniently formed as the top wall 247 of reservoir 246. The upstream and downstream sides 243 and 245 respectively of top wall 247 can be individually profiled to promote smooth flow up to and over the weir 246.

The top of weir 312 may be typically 0 to 2 inches above the level of the bottom 221 of partition walls 220/222. The concern is that the electrolyte level behind the weir be such as to maintain bottom 221 of partition walls 220/222 submerged to effect a good seal between sections 216 and 218 above the electrolyte.

The electrolyte flow over the weir 312 is preferentially stronger at the two ends of the trough to effect a flow from the centre to the ends of the trough and a sweeping flow pattern, past the weir, towards the centre of the free surface of the electrolyte in the front section. The weir can be profiled with a higher central section sloped toward lower ends, to achieve this objective. Also the spaces 313 (Figure 9) between the ends 31 1 of the weir 312 and the side walls 227 of the cell 210 favour the flow of additional electrolyte at the two sides of the metal section 218.

In a preferred configuration the forward face 255 of recovery reservoir 254 is somewhat concave toward weir 256 so that space 264 is somewhat greater in that area. The resulting increase in flow will also tend to draw the electrolyte stream toward the area of weir 256 and so to establish metal pad 318 in that area.

At a convenient location along the trough, a feeding apparatus, schematically indicated with the downward pointing arrow 314, supplies solid feed at controlled rates to the cell 210. The trough 284 is sufficiently large to effect rapid dissolution of the feed into the electrolyte without accumulation of solids on the bottom of the trough. The feeding apparatus 314 is sealed and pressurized with inert gas.

Alternatively, liquid feed as discussed later may be utilized.

The electrolyte flow is then directed towards the metal discharge weir 256 which is located as far as possible from semi-wall 220/222, usually at the centerline of symmetry (in plan view) of the cell, and slightly above the liquid level 316 in metal recovery section 218 downstream of weir 312. The separated metal is carried by the electrolyte 250 towards the weir 256 which is therefore surrounded by a metal pad 318 floating above the flowing electrolyte 250 and waiting to be discharged. In proximity to the weir 256 a metal detector 320 is positioned to detect the presence of metal floating on the electrolyte surface. Any type of metal detector or sensor can be used, but it is advantageous to use simple electric contacts, such as those used in wet bulb thermostats for the operation of domestic heating furnaces where the start/stop cycles of the furnace are activated by the contact between a mercury drop and a solid metal probe when the mercury drop moves into and out of contact with the probe by the action of the thermostat. Similarly, in the process of the present invention, the metal discharge cycles are partly controlled by the detector 320.

Preferably, the initiation of the metal discharge cycle is controlled by a clock that, at frequent intervals, starts a level rising routine, either by increasing the rate of feeding (when continuous feeding is practiced) by the feeding device 314, or by feeding inert gas into submerged open-bottom reservoir 246 shown in Figure 7. The increased electrolyte level causes the metal to overflow into submersed closed-bottom metal recovery reservoir 254, and the floating metal pad 318 becomes smaller and smaller until the detector 320 detects the absence of metal in its location and stops and reverses the level rising cycle. Detector 320 is preferably spaced from weir 256 so that when detector 320 stops the level rising cycle, there will be some metal pad remaining adjacent weir 256 and the level will not have been raised to the point where electrolyte flows into reservoir 254. If desired for increased reliability of operation, detector 320 is provided with two electrical contacts: the first to stop the level rising routine and the second to protect from accidental mis-operation of the system. After several discharge cycles the reservoir 254 becomes full and a tapping cycle must be performed through tapping port 258 before the capacity of the crucible is exceeded. To avoid overfilling, a second metal sensor 322 is preferably installed in reservoir 254. The sensor can be of the same design as sensor 320 and its function is to send a visual or audible alarm to the operator, so that he will attend to the tapping of the cell as soon as practically possible.

Typically, the tapping cycles are performed in intervals of several hours, while the clock cycles can be set at intervals of several minutes, depending on cell productivity. The actual metal discharge time could be several seconds, depending on the level control strategy being used. The level upset during metal discharge will only introduce a small disturbance to the cell operation and its effect on average cell performance will therefore be negligible.

The flow of the electrolyte 250 is now directed downwards towards a secondary settling zone 324 in the metal section 228, where the small metal droplets that have not coalesced and separated at the surface are recovered before they are recycled with the electrolyte 250 to the electrolysis section 216. The settling zone is designed to make the electrolyte meander horizontally between baffles 266 and 268 which are uniformly spaced to provide quiet paths for the electrolyte to release its residual metal droplets towards the ceiling surfaces of the baffles. Once the metal is separated from the electrolyte 250, it is easy for it to rise towards the free surface in the metal section and join the floating metal pad 318. The baffles 266 and 268 can take any convenient form. Where steel plates are used, drain holes are appropriately located in the baffle plates to allow droplets to rise.

Alternative arrangements, conventionally found to be effective in enhancing coalescence, can be used for the secondary recovery of the metal droplets. For example, an array of metal channels or inverted troughs can be positioned along the electrolyte flow path, always with the view to reduce turbulence, reduce the settling distance and in general provide additional settling surfaces for the metal droplets. Such arrangements are known in the art and are extensively used for example in oil/water separation devices. Following that practice, it may be found convenient to arrange the baffles 266 and 268 as parallel plates uniformly spaced and have the electrolyte flow disposed between them in parallel streams, all directed towards the electrolysis section, without departing from the spirit of the invention.

In Figures 7 to 10 a cell design according to the invention is shown where the baffle plates 266 and 268 are in the form of arrays of pipes or tubes 270 which are used as heat exchange surfaces. This design is particularly effective in a cell 210, such as that illustrated, where the cell walls comprise a steel casing 21 2 lined by refractory walls 214 suitable to contain the electrolyte, as compared to cells where the electrolyte is contained in a metallic crucible that can be heated or cooled externally as the case may be. The heat exchanger 272 shown in three dimensional view in Figure 10 is installed in the metal section 218 and is provided with entry and exit pipes 274 and 276 that pass through top wall/cover 309 of cell 210 and with manifolds 278. Cold air is forced through the heat exchanger 272 to cool the cell 2 0 to its operating temperature and, if desired, hot air can be used to boost the temperature up, according to the practice described in U.S. patent 4,420,381 . The new geometry affords a more efficient heat transfer and the combination of the heat transfer function with the flow streamlining function to enhance metal coalescence is a useful part of the present invention. The entry and exit pipes 274 and 276 do not need to be insulated because they can be located away from where the metal pad 318 is usually floating, avoiding as such the problem of metal freezing in contact with the pipes. The cell shown in Figure 1 1 describes an alternative embodiment of the invention that uses a submersed open-bottom metal recovery reservoir 330 which performs the same metal storing function as reservoir 254 in the previous embodiment. This is particularly effective when the metal is only slightly less dense that the electrolyte, which is the case for example for magnesium. In this case the operating principle used for the metal collection is to sweep away the metal pad 318 that tends to form on the surface of the flowing electrolyte and drag the metal down through the space 264 between the reservoir 330 and the front wall 228 of the metal section 218. In this embodiment the reservoir 330 is closed at the top centre and the weir 256 is absent. Tapping port 258 is still present. The metal separates from the electrolyte below the reservoir and is collected inside the reservoir through the open bottom 332 of the reservoir 330 itself. The metal accumulates inside the reservoir and is siphoned out from it at infrequent intervals in the conventional manner through tapping port 258.

In this embodiment electrolyte flow is not encouraged to stream toward the centre front of metal section 218, but preferably flows evenly over reservoir 330 across its width.

To facilitate the flow of metal in the Figure 1 1 embodiment, the upwardly sloped top 340 of reservoir 330 leads to increased flow velocity. Further, space 264 between reservoir 330 and front wall 228 of cell 210 is preferably reduced. Lower front edge 342 of reservoir 256 is preferably rounded and bottom wall 344 of reservoir 256 is preferably sloped upwardly toward open bottom 332 of reservoir 256. These preferred structural features all facilitate the movement of metal into the reservoir. Metal pad 346 then forms within reservoir 330 floating on electrolyte 348.

The cells shown in Figures 7 and 1 1 contain features that are desirable for intermittent feeding operations such as are used when molten feed is transported to the cell in crucibles or the like. The electrolyte volume decreases between feedings, and a compensating device in the form of a submerged open-bottom reservoir 246 is required to control the liquid level at the desired set point for optimum operation. The open-bottom reservoir 246 is supplied with controlled amounts of inert gas to compensate for electrolyte volume changes between feedings. This device is the same as provided in the prior art for the same function. The only difference is that reservoir 246 in the case of the embodiment shown in Figures 7 to 9 does not need to be operated during metal tappings, as in the prior art and as in the embodiment of Figure 1 1. Importantly, in the present invention, reservoir 246 can be used in the embodiment of Figures 7 to 9 to cycle the liquid level at pre-set time intervals to effect the metal discharge into reservoir 254 as previously described.

When the cell is fed with molten feed, it is usually done by openly discharging it into the metal recovery section. To avoid exposure of the main electrolyte surface to ambient air which reacts with the electrolyte and metal during the feeding operations, the metal recovery section 218 is maintained sealed in inert gas by providing the feed port 334 with a standpipe 336 (see Figure 8) that acts as a seal when the lid 338 is open during feeding. When the lid 338 is closed, inert gas is fed to the metal section 218 (to maintain its slight positive pressure) via the feed port 334, so that the standpipe 336 is filled with gas and therefore no metal accumulates inside it. For added freedom from freezing problems, the feeding port 334 is located away from the metal pad 318 floating on the electrolyte.

Thus, it is apparent that there has been provided in accordance with the invention a METHOD AND APPARATUS FOR ELECTROLYSING LIGHT METALS that fully satisfies the objects, aims and advantages set forth above. While the invention has been described in conjunction with (a) specific embodiment(s) thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the invention.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-/WHAT l WE CLAIM AS MY/OUR INVENTION :-

1. A process for the production of a molten metal by electrolysis in an electrolytic cell having an electrolysis section, a metal recovery section and a submerged reservoir, said process comprising: electrolysing in said electrolysis section of said cell an electrolyte containing a fused salt of said metal to produce said metal, said electrolyte having a greater density than said metal; causing said metal and additional said electrolyte to circulate continuously from said electrolysis section to said recovery section; continuously separating said metal from said electrolyte in said recovery section; causing said metal to circulate toward a part of said recovery section remote from said electrolysis section; conveying said metal from said recovery section to said submerged reservoir; and periodically recovering said metal from said reservoir.

2. A process for the production of a molten metal by electrolysis in an electrolytic cell having an electrolysis section, a metal recovery section continuous with said electrolysis section and a submerged reservoir, said process comprising: electrolysing in said electrolysis section of said cell an electrolyte containing a fused salt of said metal to produce said metal, said electrolyte having a greater density than said metal; causing said metal and additional said electrolyte to circulate continuously from said electrolysis section to said recovery section; continuously separating said metal from said electrolyte in said recovery section; causing said metal to circulate toward a part of said recovery section remote from said electrolysis section; conveying said metal from said recovery section to said submerged reservoir; and periodically recovering said metal from said reservoir.

3. A process for the production of a molten metal by electrolysis in an electrolytic cell having an electrolysis section containing a series of anodes and cathodes in facing relationship, a metal recovery section continuous with said electrolysis section, and a submerged reservoir, said process comprising: electrolysing in said electrolysis section of said cell an electrolyte containing a fused salt of said metal to produce said metal, said electrolyte having a greater density than said metal; allowing said metal and additional said electrolyte to circulate at high velocity upward in inter-electrode spaces in said electrolysis section; causing said metal and said electrolyte to circulate at low velocity between non-working faces of said cathodes and continuously to said recovery section; continuously separating said metal from said electrolyte in said electrolysis and said recovery sections; causing said metal to circulate toward a part of said recovery section remote from said electrolysis section ; conveying said metal from said recovery section to said submerged reservoir; and periodically recovering said metal from said reservoir.

4. A process for the production of a molten metal by electrolysis in an electrolytic cell having an electrolysis section, a metal recovery section, a services section, a partition extending downwardly from a top of said cell between said services section and said electrolysis and recovery sections, and a submerged reservoir extending across at least a part of said recovery section and into said services section beneath said partition wall, said process comprising: electrolysing at electrodes in said electrolysis section of said cell an electrolyte containing a fused salt of said metal to produce said metal, said salt having a greater density than said metal; rapidly circulating said electrolyte upwardly through inter electrode spaces between said electrodes; slowly circulating said electrolyte continuously from said electrolysis section toward a part of said recovery section spaced from said electrolysis section; continuously separating said metal from said electrolyte at a part of said recovery section spaced from said electrolysis section; conveying said metal from said recovery section to said submerged reservoir; and periodically recovering said metal from said reservoir through said services section.

5. The process of claim 1 wherein said cell includes a partition separating upper parts of said sections, and wherein said process further comprises maintaining said metal in said metal recovery section out of contact with said partition.

6. The process of claim 5 further comprising allowing said metal in said metal recovery section to form a pad and maintaining said paid out of contact with said partition.

7. The process of claim 6 wherein said step of conveying said metal is carried out intermittently and comprises raising the level of electrolyte in said cell to cause said pad to overflow a weir into said reservoir.

8. The process of claim 7 wherein said cell includes a level control reservoir for controlling the level of electrolyte in said cell and wherein said step of raising the level of electrolyte in said cell comprises the step of injecting gas into said level control reservoir to expel electrolyte from said reservoir to thereby raise said level in said cell.

9. The process of claim 5 wherein said reservoir is submersed in said electrolyte in said metal recovery section and includes an inlet weir extending above said electrolyte and spaced from said partition and said process further comprises directing said stream toward said inlet weir thereby to form a metal pad adjacent said weir.

10. The process of claim 5 wherein said reservoir is submersed in said electrolyte in said metal recovery section and includes an at least partially open bottom and wherein said process comprises the step of directing said stream across said recovery section above said reservoir, downwardly in said section beyond said reservoir and back under said reservoir, whereby said metal is swept under said reservoir, and allowing said metal to rise into said reservoir.

1 1. The process of claim 5, comprising the additional steps of directing said stream from a top part of said recovery section by a circuitous path downwardly through said recovery section whereby additional said metal product may coalesce and float upwardly to join said pad.

12. The process of claim 1 1 , comprising the additional steps of directing said stream from said circuitous path back toward a bottom part of said electrolysis chamber.

13. The process of claim 1 wherein said step of conveying said metal comprises directing said metal into a skimming tube located in said part of said recovery section remote from said electrolysis section.

14. The process of claim 4 wherein said step of conveying comprises pumping said metal from said recovery section into said reservoir by means of a pump.

15. An electrolytic cell comprising: an electrolysis section; a metal recovery section continuous with said electrolysis section; a submerged reservoir for storing a product metal; and means for conveying a product of electrolysis from said metal recovery section to said reservoir.

16. The cell of claim 15 comprising, in addition, a back and a front, and wherein said electrolysis section extends across said back and said recovery section extends across said front.

17. The cell of claim 15 wherein said recovery section has a bottom wall extending from a lower part of said electrolysis section to a position adjacent a surface of said electrolyte at or near said front of said cell.

18. The cell of claim 15 wherein an upper part of said bottom wall comprises a top wall of said reservoir.

19. The cell of claim 15 wherein said electrolysis section includes a series of electrodes extending from said back into said electrolysis section, said electrodes comprising a series of anodes each having a maximum cross- section adjacent said back decreasing to a minimum cross-section at their forward extremities, and a series of cathodes facing said anodes.

20. The cell of claim 19 including bipolar electrodes between said anodes and said cathodes.

21. The cell of claim 19 wherein said anodes are wedge-shaped and wherein a broad end of said wedge shape is adjacent said back of said cell and a narrow end of said wedge shape projects forwardly into said cell.

22. The cell of claim 21 wherein anode leads are mounted through said back wall of said cell.

23. The electrolytic cell of claim 15, said cell having a top and further comprising: an electrolysis section; a metal recovery section adjacent to and continuous with said electrolysis section; a services section adjacent said recovery section and remote from said electrolysis section; a partition wall extending downward from said top and separating said services section from said recovery section; a submerged reservoir in said metal recovery section for storing a product metal and extending below said partition wall into said services section; means for conveying a product of electrolysis from said metal recovery section to said reservoir; and means extending through said services section into said reservoir for periodically extracting said product metal from said reservoir.

24. The cell of claim 23 wherein said cell has a back, front and sides, said electrolysis section extends transversely across said cell adjacent said back, said recovery section extends transversely across said cell adjacent said front, and said services section is in a front corner of said cell, defined by a part of a side and said front, and wherein said partition extends diagonally across said corner.

25. The cell of claim 23 wherein said services section is exterior to the area defined by a front, back and sides of said cell and wherein said partition comprises a part of a wall of said cell.

26. The cell of claim 23 including a heat exchanger in a lower part of said cell.

27. The cell of claim 26 wherein an inlet and an outlet for said heat exchanger are located in said services section.

28. The cell of claim 26 wherein said heat exchanger is constructed for removal from said cell through said services section without disassembly of a cell cover or of said partition wall.

29. The cell of claim 26 wherein said heat exchanger is of the heat pipe type or of the thermosiphon type.

30. The cell of claim 23 including a feeding port in said services section.

31. The cell of claim 23 including a second submerged reservoir having a compressed gas inlet and outlet in a top section of said reservoir for controlling electrolyte level in said cell.

32. The cell of claim 23 wherein a front side of said reservoir is adjacent a front side of said cell.

33. The cell of claim 23 wherein said means for conveying comprises a pump.

34. The cell of claim 33 wherein said pump is powered by flow of gas into and out of a pump body, said pump including a one-way valve allowing flow in a pump inlet line only on removal of gas from said body and including a second one-way valve allowing flow in a pump outlet line only on movement of gas into said pump body.

35. The cell of claim 23 wherein said means extending into said reservoir comprises one end of a syphon tube.

36. The cell of claim 23 wherein said means for periodically extracting comprises means for delivering said product metal to a pipeline loop, and wherein said cell further comprises means for electrically insulating said cell from said pipeline loop except during delivery of said product metal to said pipeline loop.

37. The cell of claim 1 5 further comprising a first partition wall separating a top part of said electrolysis section from a top part of said metal recovery section and extending slightly down into a top surface of said electrolyte for forming a gas seal between said sections.

38. The cell of claim 37, containing a second said partition wall spaced from said first partition wall.

39. The cell of claim 38 wherein said space between said partition walls leads out of said cell and is connected to gas scrubbing apparatus.

40. The cell of claim 37, including in said metal recovery section a first weir extending substantially across said section conforming generally to the direction of said partition wall.

41 . The cell of claim 40 wherein the ends of said weir are spaced from adjacent sidewalls of said cell.

42. The cell of claim 40 wherein said weir is profiled to direct a greater proportion of flow toward and across its ends.

43. The cell of claim 40 wherein at least one of the front and back faces of said weir is gently sloped to facilitate smooth flow over said weir.

44. The cell of claim 41 wherein both said faces are so sloped.

45. The cell of claim 40, including an open bottom level control reservoir in said metal recovery section and positioned to be immersed in electrolyte during normal operation of said cell.

46. The cell of claim 45 in which a top surface of said level control reservoir is profiled to form said weir.

47. The cell of claim 37, including a trough running the length of said first partition wall and spaced below said wall, the sides of said trough located respectively in said electrolysis section and said metal recovery section.

48. The cell of claim 47 wherein said side of said trough in said electrolysis section is formed in part by a cathode, said wall of said trough in said metal recovery section is formed by said weir and the bottom of said trough is formed by a generally horizontal partition between said cathode and said weir.

49. The cell of claim 41 , including a recovery reservoir.

50. The cell of claim 37, including a recovery reservoir.

51. The cell of claim 49 wherein said recovery reservoir is located in said recovery section and positioned to be immersed in electrolyte during normal operation of said cell.

52. The cell of claim 51 , including an inlet to said recovery reservoir extending to a position above the electrolyte to thereby form an inlet weir to said recovery reservoir.

53. The cell of claim 52, including a timing device for periodically activating means causing inert gas to flow into said level control reservoir during operation of said cell to raise the level of electrolyte in said cell to thereby cause metal adjacent said inlet weir to flow over said inlet weir and into said recovery reservoir.

54. The cell of claim 53 further including metal detection means adjacent said inlet weir for detecting the absence of metal adjacent said weir and responsive thereto for activating means causing inert gas to be removed from said level control reservoir to lower the level of electrolyte in said cell to prevent overflow of electrolyte into said recovery reservoir.

55. The cell of claim 49, including in said recovery reservoir a level sensor for sensing the level of metal in said reservoir during operation of said cell and for providing a signal responsive to metal reaching preselected level or levels.

56. The cell of claim 51 wherein a side of said recovery reservoir remote from said partition wall is spaced a short distance from a front wall of said cell.

57. The cell of claim 51 wherein said recovery reservoir has an at least partially open bottom.

58. The cell of claim 57, having a rounded lower front edge.

59. The cell of claim 57, having a part of a bottom wall sloped upwardly in a front to rear direction toward said open bottom.

60. The cell of claim 56 wherein said metal recovery section includes at least one baffle extending from a position adjacent a front wall of said cell to a position within said section to provide a circuitous path for electrolyte flow vertically in said section.

61. The cell of claim 60, including at least two said baffles vertically spaced from each other.

62. The cell of claim 61 wherein each said baffle is comprised of a series of heat exchanger tubes comprising part of a heat exchanger positioned in said metal recovery section.

63. The cell of claim 51 , including at least one additional baffle which together with said baffles defines a circuitous path vertically within said metal recovery section.