Title:
Electrolytic process for the production of metals in molten halide systems
United States Patent 3909375
Abstract:
Metals are produced by electrolysis of their respective metal halides wherein the metals are deposited on one of a pair of spaced substantially parallel electrodes, the opposed surfaces of which are inclined at an angle of between 5° and 30° to the vertical, and wherein gas liberated in the inter-electrode space is discharged upwardly into a gas separation chamber disposed above the inter-electrode space.
US Patent References:
Apparatus for electrolyzing fused salts
Ashcroft - July 1925 - 1545383
Producing or refining aluminum
Ransley - April 1962 - 3028324
Furnace for fused-bath electrolysis, particularly for aluminum production from alo
De Pava - December 1962 - 3067124
Electrolytic cell for production of aluminum and method of making the same
Lewis et al. - September 1968 - 3400061
LIGHT METAL PRODUCTION
Haupin - August 1973 - 3755099
Apparatus for electrolyzing fused salts
Ashcroft - July 1925 - 1545383
Producing or refining aluminum
Ransley - April 1962 - 3028324
Furnace for fused-bath electrolysis, particularly for aluminum production from alo
De Pava - December 1962 - 3067124
Electrolytic cell for production of aluminum and method of making the same
Lewis et al. - September 1968 - 3400061
LIGHT METAL PRODUCTION
Haupin - August 1973 - 3755099
Inventors:
Holliday, Robin David (Daytona Beach, FL)
Mcintosh, Peter (Gladstone, AU)
Mcintosh, Peter (Gladstone, AU)
Application Number:
05/349472
Publication Date:
09/30/1975
Filing Date:
04/09/1973
View Patent Images:
Export Citation:
Assignee:
Conzinc Riotinto of Australia Limited (Melbourne, AU)
Primary Class:
Other Classes:
204/270, 204/247.400, 205/391, 205/381, 204/247
International Classes:
C25C7/00; C25C3/22; C25C3/00; C25C3/06; C25C3/04
Field of Search:
204/67,70,243 R-247/ 204/64R
Primary Examiner:
Mack, John H.
Assistant Examiner:
Weisstuch, Aaron
Attorney, Agent or Firm:
Larson, Taylor And Hinds
Claims:
We claim
1. In the operation of an electrolytic cell for use in the electrolytic production of metals in molten halide systems, which comprises a pair of closely spaced substantially parallel planar non-consumable electrodes inclined at an angle of between 7 and 15 degrees to the vertical, an interelectrode spacing for said molten halide system, the cathode surface being inclined at a positive angle to the vertical and the anode surface being inclined at a similar negative angle to the vertical, the inter-electrode spacing being less than two inches, and a gas separation chamber disposed above and communicating with the inter-electrode space into which gas is discharged upwardly from the inter-electrode space, the area of the melt surface of said molten halide system in the gas separation chamber and the depth of the electrolyte in the gas separation chamber being sufficient to permit separation of gas from the electrolyte in the gas separation chamber at substantially the same rate as it is produced in the inter-electrode space, the width of the melt surface in the gas separation chamber being not less than ##EQU2##
2. L is the cathode length in inches
3. M is the inter-electrode spacing in inches, and the width of the melt surface and the depth of the electrolyte in the gas separation chamber being each not less than twice the inter-electrode spacing.
4. In the operation according to claim 1 wherein the width of the melt surface is not greater than the depth of the electrolyte in the gas separation chamber.
5. An electrolytic cell according to claim 1 wherein the width of the melt surface and the depth of the electrolyte in the gas separation chamber are each not less than 4 inches.
6. An electrolytic cell according to claim 1 wherein the inter-electrode spacing is between 1.2 and 1.8 inches.
7. An electrolytic cell according to claim 1 and having a plurality of pairs of electrodes arranged in parallel.
8. An electrolytic cell according to claim 1 wherein a pool of molten metal is formed below the electrodes but said pool does not serve as a cathode on which metal is deposited.
9. An electrolytic cell according to claim 1 wherein the electrolysis is carried out at a current density of not less than 1 amp/cm2.
1. In the operation of an electrolytic cell for use in the electrolytic production of metals in molten halide systems, which comprises a pair of closely spaced substantially parallel planar non-consumable electrodes inclined at an angle of between 7 and 15 degrees to the vertical, an interelectrode spacing for said molten halide system, the cathode surface being inclined at a positive angle to the vertical and the anode surface being inclined at a similar negative angle to the vertical, the inter-electrode spacing being less than two inches, and a gas separation chamber disposed above and communicating with the inter-electrode space into which gas is discharged upwardly from the inter-electrode space, the area of the melt surface of said molten halide system in the gas separation chamber and the depth of the electrolyte in the gas separation chamber being sufficient to permit separation of gas from the electrolyte in the gas separation chamber at substantially the same rate as it is produced in the inter-electrode space, the width of the melt surface in the gas separation chamber being not less than ##EQU2##
2. L is the cathode length in inches
3. M is the inter-electrode spacing in inches, and the width of the melt surface and the depth of the electrolyte in the gas separation chamber being each not less than twice the inter-electrode spacing.
4. In the operation according to claim 1 wherein the width of the melt surface is not greater than the depth of the electrolyte in the gas separation chamber.
5. An electrolytic cell according to claim 1 wherein the width of the melt surface and the depth of the electrolyte in the gas separation chamber are each not less than 4 inches.
6. An electrolytic cell according to claim 1 wherein the inter-electrode spacing is between 1.2 and 1.8 inches.
7. An electrolytic cell according to claim 1 and having a plurality of pairs of electrodes arranged in parallel.
8. An electrolytic cell according to claim 1 wherein a pool of molten metal is formed below the electrodes but said pool does not serve as a cathode on which metal is deposited.
9. An electrolytic cell according to claim 1 wherein the electrolysis is carried out at a current density of not less than 1 amp/cm2.
Description:
This invention relates to a process for the production of metals, particularly aluminium and magnesium, by electrolysis of their respective metal halides, preferably chlorides, contained in alkali halide melts, preferably alkali chloride melts, and the invention also refers to electrolytic cells for use in such process.
Cells of the type discussed are not restricted to production of aluminium and magnesium, but can be applied in any system where the metal product is heavier than the solvent electrolyte, and is formed as a liquid phase -- for example, lead, bismuth, zinc, cerium, gallium -- from the respective molten halide (preferably chloride) solvents.
In one form, the invention may comprise a system for the production of aluminium or magnesium from their respective halide salts wherein molten metal accumulates on the bottom of an electrolytic cell, as in conventional reduction technology, but where the mobile molten pool of metal does not necessarily serve as cathode on which metal is deposited, or through which current is withdrawn from the cell. Instead, metal is preferably deposited on non-consumable electrodes which are inclined to the vertical at relatively small angles, for example, 5° to 30°, and which are substantially parallel to the opposite electrodes. Such non-consumable electrode sheets may be connected alternately to the positive and negative poles of the power supply and hence may constitute a system of electrodes in parallel.
In another form of the invention, only the end members of a system of parallel sheets need be connected to the power supply and the intervening sheets would then be made to function as bipolar electrodes.
It is not necessary, and is in fact undesirable, that anything but a small portion of the electric current should pass through the pool of metal accumulated at the bottom of the cell.
An important feature of the invention is the provision made for facilitating gas, e.g. chlorine, to be liberated from the interelectrode space during electrolysis by means of a suitable gas separation chamber so that said gas is substantially prevented from "back reacting" with metal in the vicinity of the cathode surface.
The invention comprises a process for the electrolytic production of metals in molten halide systems, wherein metal is deposited on one of a pair of spaced substantially parallel electrodes, the opposed surfaces of which are inclined at an angle of between 5° and 30° to the vertical, and wherein gas liberated in the inter-electrode space is discharged upwardly into a gas separation chamber disposed above the inter-electrode space.
Preferably the cathode surface on which the metal is deposited is inclined at a positive angle to the vertical and the spaced anode surface is inclined at a similar negative angle to the vertical. The electrodes are preferably planar, non-consumable and closely spaced. The opposed surfaces of the electrodes are preferably inclined at an angle of between 7° and 20° to the vertical.
The area of the melt surface and the depth of the liquid electrolyte in the gas separation chamber are preferably sufficient to permit separation of gas from the electrolyte in the gas separation chamber at substantially the same rate as said gas is produced in the inter-electrode space.
The following is a brief description of the drawings:
FIG. 1 is a highly diagrammatic representation of a gas separation chamber used in explanation of two critical parameters in such a chamber;
FIGS. 2 and 3 are diagrammatic side-elevational views of two different cell constructions used in illustrating the differences in operation of two different cell configurations;
FIG. 4 is a diagrammatic side elevational view of a multi-electrode cell according to the invention; and
FIG. 5 is a diagrammatic side elevational view of an alternative embodiment of the invention employing a more compact electrode configuration.
More particularly, the invention in one form relates to cell designs which take maximum advantage of the compactness made possible by using a system of closely spaced planar electrodes inclined at relatively small angles (e.g. 5° to 30°) to the vertical, and operating at high current densities, e.g. in excess of 1 amp/cm 2 (amps per sq.cm), preferably not less than 1.5 amp/cm 2 . The inter-electrode spacing (A.C.D.) is preferably less than 2 inches and preferably between 1.2 and 1.8 inches. In any given case, the angle at which the electrodes are inclined will vary with the normal operating parameters; for example, it may be expected that the angles of inclination will be greater for higher current densities and for smaller interelectrode spacings (A.C.D's). The invention thus enables the attainment of significant advantages in respect of operating and capital costs.
In the case of aluminium, reduction cell size can be reduced so that steel and refractory requirements are of the order of a quarter of those for conventional cells of the same productive capacity, and the floor area required can be reduced to one-fifth. Typical cell dimensions for possible electrode configurations obtained by scaling up results obtained from the experiments referred to in the Examples which follow, are shown in Table I below:
TABLE I ____________________________________________________________ ______________ Super- Working ficial Floor depth of Cell Cell Cell area area Cell Type Number of Electrodes length width height (square (square and Capacity Electrodes (inches) (feet) (feet) (feet) feet) feet) ____________________________________________________________ ______________ Monopolar 150,000 amp 3 36 6.0 7.5 5.0 126 45 Monopolar 150,000 amp 2 36 9.6 4.5 5.0 170 53 Monopolar 150,000 amp 4 24 7.3 6.9 4.0 148 50 Bipolar 37,500 amp 2 24 13.5 4.0 4.0 159 54 Bipolar 25,000 amp 3 24 9.0 5.5 4.0 123 50 ____________________________________________________________ ______________ NOTE: Bipolar cells have similar production capacities to monopolar cells but operate at lower cell currents.
Because of the simplified design, and greatly reduced size, of cells constructed according to this invention, it is calculated that the capital cost could be reduced to about a quarter of that for conventional cells at the 150,000 amp level, and total electrolysis plant costs could be reduced to about one-third that of conventional plant.
With magnesium production the cost of cells using low density electrolytes, may be similarly reduced by a factor of 2 to 3 compared with cell designs used in the conventional processes, or with Hall-type magnesium cells using comparable low density electrolytes.
A significant operating advantage of the dry vertical electrode geometry of this invention, in general, is the removal of the restriction on cell current or current density, which is always imposed on conventional liquid cathode cells by the magnetic stirring effects of the large currents. The removal of this restriction by elimination of the liquid cathode and by the improved cell geometry of this invention, means that cells of several hundred thousand amperes capacity are now brought within the range of practicality. In addition, stable values of inter-electrode spacing are maintained without the need for manipulating the electrodes or adjusting the level of the pool of metal accumulated at the base of the cell.
Because of the stability of inter-electrode distance and the lack of disturbance by magnetic stirring it becomes possible by means of this invention to operate chloride reduction cells with inclined cathodes at considerably lower inter-electrode distances than are practicable in conventional cells. Since a significant proportion of the energy consumed in conventional electrolytic cells is dissipated as heat (due to resistance effects) the new electrode geometry of this invention makes possible not only savings in electrical energy but also a simplification of the problem of removing heat from large cells.
We have found that an important condition for successfully operating cells with close-packed inclined electrodes at high current density is the provision of an adequately designed gas separation chamber. Models of the gas-liquid flow patterns produced by the anode reaction have shown that it is necessary to provide sufficient depth of liquid electrolyte and sufficient liquid-gas interfacial area above the cathode to permit complete separation of gas from the electrolyte at substantially the same rate as it is produced in the anode reaction. The gas pumping effect of the anode reaction produces vortexes in the vicinity of the melt-gas interface. If the rate of evolution of gas from the melt is too low because of inadequate free surface area, then gas that has not escaped may continue to circulate in the vortex pattern where it will accumulate and cause a froth layer to form and to increase in thickness until it extends into the region between the electrodes. In addition, when the rate of gas evolution, in other words current density, is increased in a cell of given A.C.D., a point is reached where gas becomes entrained in melt returning to the inter-electrode region.
Inter-electrode distance (A.C.D.) has an additional important effect because melt returning to the inter-electrode space after having been gas-pumped to the surface may interact with the ascending stream of gas and liquid. This causes gas to be diverted from the upward stream and re-directed down into the interelectrode space. For a given rate of gas evolution, i.e. current density, increasing the A.C.D. will eliminate this effect. For a given total rate of gas evolution there thus exists an optimum A.C.D. which represents the best compromise between avoidance of gas recirculation and increase of cell voltage because of the increase of current path. For current densities in the vicinity of 1.5 amp/cm 2 and with electrodes up to 24 inches in working length inclined at 10° to the vertical an A.C.D. of 1.5 inches has been found to be near optimum.
Two critical parameters of the gas separation chamber are the width of the melt surface (i.e. the surface between the liquid electrolyte and gas) in the gas separation chamber, and the depth of liquid electrolyte in the gas separation chamber above the cathode. These two parameters are shown in FIG. 1 of the accompanying drawings as S and D respectively. In FIG. 1, the numeral 1 indicates the anode, 2 indicates the cathode, 5 indicates the liquid electrolyte in the gas separation chamber above the cathode 2, 8a indicates the quiescent melt level, S indicates the width of the melt surface, D indicates the depth of the electrolyte in the gas separation chamber above the cathode, L indicates the cathode length, and M indicates the A.C.D. (inter-electrode distance).
Our model studies suggest that, for electrodes inclined at between 7° to 15° to the vertical, conservative or minimum values for S may be attained by applying the empirically derived formula: ##EQU1## where S is in inches, C is the numerical value of the current density in amps/cm 2 , L is the numerical value of the cathode length in inches, and M is the numerical value of the inter-electrode spacing (A.C.D.) in inches. The width of the melt surface and the depth of the liquid electrolyte in the gas separation chamber are preferably not less than twice the inter-electrode spacing, and are preferably not less than four inches. Preferably, S is not greater than D.
There is evidence of the use of vertical electrodes in cells for the production of both aluminium and magnesium, e.g. in U.S. Pat. No. 2,512,157 (Johnson). The Johnson cell, however, is limited to refining impure solid aluminium anodes in chloride electrolyte with deposition of solid aluminium on aluminium cathodes. The lack of provision for adequate gas liberation from between the electrodes would result in low current efficiencies if this cell were adapted for electrowinning of aluminium.
Prior aluminium extraction processes, such as, for example those covered by U.S. Pat. Nos. 2,959,533 and 3,382,166 (de Varda) and 3,352,767 (de Garab et al.), operate with variable A.C.D. and also do not provide adequately for separation of gas. These patents also specify the use of the conventional cryolite-Al 2 O 3 electrolyte, including the consumable carbon anodes which are mandatory in such a system.
In the case of magnesium, the conventional electrolytic cells used for the reduction of magnesium chloride employ vertical anodes in conjunction with high density electrolytes, i.e. electrolytes which are heavier than the molten magnesium produced, and complex cathode designs are thus necessary to collect the molten magnesium at the surface of the melt. Although low-density chloride electrolytes which enable magnesium to be collected on the cell floor are now known, the existing cell designs for use with such electrolytes are quite different from the novel and efficient cell designs proposed in this invention.
U.S. Pat. Nos. 2,468,022 and 2,696,688 (Blue et al.) describe a bipolar electrode magnesium cell of considerable complexity which employs vertical electrodes, but differs from the present invention in a number of important ways. The objectives of the Blue design are to simplify electrolysis chamber construction, to provide improved gas sealing, and to increase the capacity of the cells by providing a larger number of electrodes than in the conventional apparatus. However, these objectives are achieved at considerable cost in complicating the overall cell structure. Thus, a separate two or three compartment feed chamber together with a molten salt distributing system consisting either of a weir chamber mounted above the centre of the cell or a series of distributing channels incorporated into the walls of the cell are required. In addition, a molten salt pump for circulation of the electrolyte is needed.
The Blue cell relies upon forced circulation of melt to remove magnesium from the inter-electrode space in cells which use melt systems of higher density than magnesium. In these cells the molten metal floats to the surface. The cell is said to operate at 0.43 to 0.45 amp/cm 2 , typical of conventional practice. However, current efficiency is said to be about 75 percent, and it is to be noted that no actual operating conditions are described. The low current efficiency is not surprising, because the design is believed to be quite inadequate for securing separation of chlorine from magnesium. Considerable entrapment of gas between the electrodes must occur because not only are the electrodes devoid of any slope, but the inter-electrode gap is also narrow right up to and beyond the surface of the melt.
The cell of Blue et al. does not provide anything approaching the compactness and efficiency of the cell of the present invention, because of the use of low current density and more particularly the lack of provision for adequate chlorine removal, which leads to low current efficiency. Further, the expense of the additional components is likely to nullify to a large extent the claimed advantage of being able to incorporate more electrodes at lower A.C.D. within the electrolysis chamber than in conventional plant.
U.S. Pat. No. 3,396,094 (Sivilotti et al.) illustrates the lengths to which it is necessary to go to improve current efficiency in conventional cells by modifying the method of collecting magnesium at the melt surface. U.S. Pat. No. 3,418,223 (Love) shows the waste of space within a cell resulting from the use of a two-piece cathode for collecting metal at the surface of the melt and also shows the structural complexity of the typical chlorine collection chamber. In both Sivilotti's and Love's cells, separation of chlorine from the melt may be expected to be substantially incomplete because of the lack of adequate free surface for gas separation.
An important feature of one form of our invention is that the cathode surface is inclined at a positive angle, e.g. plus 5° to 30°, to the vertical and faces a parallel or substantially parallel planar anode inclined at a negative angle, e.g. minus 5° to 30°, to the vertical. In other words, it is important in the system of this form of the invention that the working surface of the cathode is arranged to face slightly upwards and the corresponding surface of the anode is arranged to face slightly downwards.
The inter-electrode spacing is preferably less than 2 inches, and desirably between 1.2 and 1.8 inches. As discussed in detail above, study of melt circulation paths leads to the preferred specification of an anode-cathode spacing of about 1.5 inches at current densities of 1.5 amp/cm 2 to 2.0 amp/cm 2 .
Other parameters of importance in the design of the gas separation chamber are the electrolyte depth above the cathode and the width or area of the surface of the electrolyte in the gas separation chamber. It was found that provision of a gas separation chamber extending back 4 to 5 inches from the anode shoulder, across the inter-electrode gap and extending over the top of the cathode structure, was adequate for 12-inch cathodes. For other values of the cell parameters, Formula A referred to above is applicable, within the range of electrode inclinations stated.
The very considerable and somewhat unexpectedly beneficial effect of the electrode arrangement of this invention is demonstrated by comparison of FIGS. 2 and 3 of the accompanying drawings which illustrate the operation of a less favourable and more favourable cell configuration, respectively.
In FIGS. 2 and 3 the numeral 1 indicates the anode having active anode surfaces 1a, 2 indicates the cathodes having active cathode surfaces 2a, 8 indicates the electrolyte, 8a the upper surface of the electrolyte, and 10 the inter-electrode space.
A indicates regions of severe gas formation and B indicates regions of less severe gas formation. In FIG. 3, 9 is the gas separation chamber immediately above the cathode 2.
In FIGS. 2 and 3 the inclination of the active electrode surfaces to the vertical was about 10° and the inter-electrode distance (A.C.D.) was 1.5 inches. In FIG. 2 the current density was 1 amp/cm 2 and in FIG. 3 it was 2 amp/cm 2 . In FIG. 2 the depth of the quiescent electrolyte above the working cathode surface, within the inter-electrode space, was 2 inches; and in FIG. 3 the depth of the quiescent electrolyte above the working cathode surface, within the gas separation chamber, was 4 inches. The width of the upper surface of the electrolyte was 1.5 inches in FIG. 2 and 4 inches in FIG. 3.
It will be seen that retention of gas between the electrodes is very severe under the conditions shown in FIG. 2, even at a moderately low current density of 1 amp/cm 2 . Operation under this condition is found to result in back reaction of about 30 per cent of product, in other words a current efficiency of 70 per cent or less.
FIG. 3 illustrates the effect of electrolysis using a cell design and gas separation chamber constructed in accordance with one form of this invention. Operation with improved gas liberation under the conditions shown in FIG. 3 raised the current efficiency to about 90 percent.
A feature of the form of the invention shown in FIG. 3, as compared with the cell design shown in FIG. 2, is that the width and depth of the gas separation chamber 9 immediately above the working cathode surface 2a are sufficient to ensure (a) that the gas formed in the inter-electrode space 10 during electrolysis is discharged or liberated to a substantial degree from such space 10 into the gas separation chamber 9, and (b) that said gas is liberated to a substantial degree from the electrolyte in said gas separation chamber 9. Preferably the width and/or depth of the gas separation chamber 9 are at least twice the inter-electrode distance (A.C.D.). The above-described cell arrangement substantially reduces the back reaction which would otherwise occur between the gas and the metal in the vicinity of the cathode surface 2a (as indicated in FIG. 2) and thus substantially increases the current efficiency of the cell.
FIG. 4 represents a diagrammatic side elevation of a type of multi-electrode cell designed according to the present invention. Overall dimensions of the cells are preferably as indicated in Table I. 1 designates the graphite anodes with electrolytically active surfaces 1a inclined at a negative angle of about 10° to the vertical, and provided with recesses 4 to form a gas liberation chamber 5 which is preferably proportioned according to Formula A and which affords an adequate rate of gas evolution from the melt surface 8a. 2 designates the cathodes, which for aluminium production are of graphite and for magnesium production may be hollow fabricated steel structures or plain steel sheets, having cathode surfaces 2a which are at a positive angle of about 10° to the vertical and are substantially parallel to the anode surfaces 1a, and 3 represents the refractory-lined steel shell. 8 designates the electrolyte and 6 the electrical current connections for the anodes. Connections for the cathodes 2 are not shown; these may, if desired, be made directly to the shell 3. It will be understood that in any cell configuration the anode(s) may be adjusted in a vertical or substantially vertical direction for setting or re-setting of A.C.D.
FIG. 5 represents a diagrammatic side elevation of a more compact electrode configuration, namely that employing bipolar electrodes. The connotations of the designating numerals used are not the same as in FIG. 4. 1 designates the graphite anode, with working surfaces 1a inclined according to the invention, 2 the bipolar electrodes with working surfaces 2a, which electrodes 2 may be monolithic graphite blocks supported at their ends by the insulating walls of the cell. 3 designates the collector cathodes, which may be of steel in the case of magnesium, but of graphite in the case of aluminium cells, and 3a represents the working surfaces of the cathodes 3. 4 represents the refractory lined outer steel shell and 5 the gas liberation chamber, preferably proportioned according to Formula A. The insulating lower supports 6 for the bipolar electrodes 2 serve as barriers for reducing leakage current.
EXAMPLE 1
A run was carried out in a cell having a single inclined anode, and of the general form shown in FIG. 3. The effective electrode area was approximately 1000 cm 2 . Electrolyte composition was 21% MgCl 2 -75% KCl-4%LiCl, and a total cell current of 400 amps was used, at a temperature of 850°C.
The anode-cathode slope was 9° to the vertical and was within the recommended range for efficient operation. The other parameters were chosen to test some of the less favourable conditions for gas release. The most adverse feature was the use of a melt depth of only 1.5 ins. above the cathode, together with a current density of only 0.36 amp/cm 2 . Under these conditions, considerable back reaction was anticipated to occur because of back circulation of chlorine into the inter-electrode space.
During a run of 61 minutes duration, 404 g of Cl 2 were produced and 543 g of MgCl 2 were consumed.
The current efficiency was 75 percent.
EXAMPLE 2 -- Magnesium
A run was carried out using a single anode cell of the general form shown in FIG. 3 and having an effective electrode area of approximately 1000 cm 2 .
The electrolyte composition was 21% MgCl 2 , 75% K Cl and 4% LiCl.
The anode/cathode slope was 9° to the vertical and was within the recommended range for efficient operation. The cathode length L was 12 inches, S and D were each 4 inches. The operating temperature was 850°C, current density was 0.64 amp/cm 2 and a total cell current of 700 amps was maintained during the 60 minute duration of the run.
801 g of Cl 2 were produced and 1077 g of MgCl 2 were consumed so that current efficiency was 88 percent.
EXAMPLE 3 -- Magnesium
A second run was carried out in the same cell as in Example 2 using an electrolyte containing 22% MgCl 2 , 29% KCl, and 50% LiCl. Operating conditions were chosen to demonstrate one of the optimum combinations of parameters attainable in a cell model.
Current density was increased to 1.5 amp/cm 2 and the cell was operated with a melt depth of 4 in. over the cathode at a temperature of 850°. L was 12 inches and S and D were each 4 inches. During the 45 minute run a steady cell current of 1650 amps was used.
1477 g of Cl 2 were recovered, and 1981 g of MgCl 2 were consumed so that the current efficiency was 90 percent.
EXAMPLE 4 -- Aluminium
A run was carried out in the cell of the general form shown in FIG. 3 using a melt composition averaging 10% Al Cl 3 , 45% NaCl, 45% KCl. The dimensions of the cell were as stated in Example 3. The temperature was 730°C. During the run of 1 hour the cell current was 1400 amps.
1857 g of Cl 2 were recovered and 2323 g of Al Cl 3 were consumed.
The average current efficiency was 89 percent.
Cells of the type discussed are not restricted to production of aluminium and magnesium, but can be applied in any system where the metal product is heavier than the solvent electrolyte, and is formed as a liquid phase -- for example, lead, bismuth, zinc, cerium, gallium -- from the respective molten halide (preferably chloride) solvents.
In one form, the invention may comprise a system for the production of aluminium or magnesium from their respective halide salts wherein molten metal accumulates on the bottom of an electrolytic cell, as in conventional reduction technology, but where the mobile molten pool of metal does not necessarily serve as cathode on which metal is deposited, or through which current is withdrawn from the cell. Instead, metal is preferably deposited on non-consumable electrodes which are inclined to the vertical at relatively small angles, for example, 5° to 30°, and which are substantially parallel to the opposite electrodes. Such non-consumable electrode sheets may be connected alternately to the positive and negative poles of the power supply and hence may constitute a system of electrodes in parallel.
In another form of the invention, only the end members of a system of parallel sheets need be connected to the power supply and the intervening sheets would then be made to function as bipolar electrodes.
It is not necessary, and is in fact undesirable, that anything but a small portion of the electric current should pass through the pool of metal accumulated at the bottom of the cell.
An important feature of the invention is the provision made for facilitating gas, e.g. chlorine, to be liberated from the interelectrode space during electrolysis by means of a suitable gas separation chamber so that said gas is substantially prevented from "back reacting" with metal in the vicinity of the cathode surface.
The invention comprises a process for the electrolytic production of metals in molten halide systems, wherein metal is deposited on one of a pair of spaced substantially parallel electrodes, the opposed surfaces of which are inclined at an angle of between 5° and 30° to the vertical, and wherein gas liberated in the inter-electrode space is discharged upwardly into a gas separation chamber disposed above the inter-electrode space.
Preferably the cathode surface on which the metal is deposited is inclined at a positive angle to the vertical and the spaced anode surface is inclined at a similar negative angle to the vertical. The electrodes are preferably planar, non-consumable and closely spaced. The opposed surfaces of the electrodes are preferably inclined at an angle of between 7° and 20° to the vertical.
The area of the melt surface and the depth of the liquid electrolyte in the gas separation chamber are preferably sufficient to permit separation of gas from the electrolyte in the gas separation chamber at substantially the same rate as said gas is produced in the inter-electrode space.
The following is a brief description of the drawings:
FIG. 1 is a highly diagrammatic representation of a gas separation chamber used in explanation of two critical parameters in such a chamber;
FIGS. 2 and 3 are diagrammatic side-elevational views of two different cell constructions used in illustrating the differences in operation of two different cell configurations;
FIG. 4 is a diagrammatic side elevational view of a multi-electrode cell according to the invention; and
FIG. 5 is a diagrammatic side elevational view of an alternative embodiment of the invention employing a more compact electrode configuration.
More particularly, the invention in one form relates to cell designs which take maximum advantage of the compactness made possible by using a system of closely spaced planar electrodes inclined at relatively small angles (e.g. 5° to 30°) to the vertical, and operating at high current densities, e.g. in excess of 1 amp/cm 2 (amps per sq.cm), preferably not less than 1.5 amp/cm 2 . The inter-electrode spacing (A.C.D.) is preferably less than 2 inches and preferably between 1.2 and 1.8 inches. In any given case, the angle at which the electrodes are inclined will vary with the normal operating parameters; for example, it may be expected that the angles of inclination will be greater for higher current densities and for smaller interelectrode spacings (A.C.D's). The invention thus enables the attainment of significant advantages in respect of operating and capital costs.
In the case of aluminium, reduction cell size can be reduced so that steel and refractory requirements are of the order of a quarter of those for conventional cells of the same productive capacity, and the floor area required can be reduced to one-fifth. Typical cell dimensions for possible electrode configurations obtained by scaling up results obtained from the experiments referred to in the Examples which follow, are shown in Table I below:
TABLE I ____________________________________________________________ ______________ Super- Working ficial Floor depth of Cell Cell Cell area area Cell Type Number of Electrodes length width height (square (square and Capacity Electrodes (inches) (feet) (feet) (feet) feet) feet) ____________________________________________________________ ______________ Monopolar 150,000 amp 3 36 6.0 7.5 5.0 126 45 Monopolar 150,000 amp 2 36 9.6 4.5 5.0 170 53 Monopolar 150,000 amp 4 24 7.3 6.9 4.0 148 50 Bipolar 37,500 amp 2 24 13.5 4.0 4.0 159 54 Bipolar 25,000 amp 3 24 9.0 5.5 4.0 123 50 ____________________________________________________________ ______________ NOTE: Bipolar cells have similar production capacities to monopolar cells but operate at lower cell currents.
Because of the simplified design, and greatly reduced size, of cells constructed according to this invention, it is calculated that the capital cost could be reduced to about a quarter of that for conventional cells at the 150,000 amp level, and total electrolysis plant costs could be reduced to about one-third that of conventional plant.
With magnesium production the cost of cells using low density electrolytes, may be similarly reduced by a factor of 2 to 3 compared with cell designs used in the conventional processes, or with Hall-type magnesium cells using comparable low density electrolytes.
A significant operating advantage of the dry vertical electrode geometry of this invention, in general, is the removal of the restriction on cell current or current density, which is always imposed on conventional liquid cathode cells by the magnetic stirring effects of the large currents. The removal of this restriction by elimination of the liquid cathode and by the improved cell geometry of this invention, means that cells of several hundred thousand amperes capacity are now brought within the range of practicality. In addition, stable values of inter-electrode spacing are maintained without the need for manipulating the electrodes or adjusting the level of the pool of metal accumulated at the base of the cell.
Because of the stability of inter-electrode distance and the lack of disturbance by magnetic stirring it becomes possible by means of this invention to operate chloride reduction cells with inclined cathodes at considerably lower inter-electrode distances than are practicable in conventional cells. Since a significant proportion of the energy consumed in conventional electrolytic cells is dissipated as heat (due to resistance effects) the new electrode geometry of this invention makes possible not only savings in electrical energy but also a simplification of the problem of removing heat from large cells.
We have found that an important condition for successfully operating cells with close-packed inclined electrodes at high current density is the provision of an adequately designed gas separation chamber. Models of the gas-liquid flow patterns produced by the anode reaction have shown that it is necessary to provide sufficient depth of liquid electrolyte and sufficient liquid-gas interfacial area above the cathode to permit complete separation of gas from the electrolyte at substantially the same rate as it is produced in the anode reaction. The gas pumping effect of the anode reaction produces vortexes in the vicinity of the melt-gas interface. If the rate of evolution of gas from the melt is too low because of inadequate free surface area, then gas that has not escaped may continue to circulate in the vortex pattern where it will accumulate and cause a froth layer to form and to increase in thickness until it extends into the region between the electrodes. In addition, when the rate of gas evolution, in other words current density, is increased in a cell of given A.C.D., a point is reached where gas becomes entrained in melt returning to the inter-electrode region.
Inter-electrode distance (A.C.D.) has an additional important effect because melt returning to the inter-electrode space after having been gas-pumped to the surface may interact with the ascending stream of gas and liquid. This causes gas to be diverted from the upward stream and re-directed down into the interelectrode space. For a given rate of gas evolution, i.e. current density, increasing the A.C.D. will eliminate this effect. For a given total rate of gas evolution there thus exists an optimum A.C.D. which represents the best compromise between avoidance of gas recirculation and increase of cell voltage because of the increase of current path. For current densities in the vicinity of 1.5 amp/cm 2 and with electrodes up to 24 inches in working length inclined at 10° to the vertical an A.C.D. of 1.5 inches has been found to be near optimum.
Two critical parameters of the gas separation chamber are the width of the melt surface (i.e. the surface between the liquid electrolyte and gas) in the gas separation chamber, and the depth of liquid electrolyte in the gas separation chamber above the cathode. These two parameters are shown in FIG. 1 of the accompanying drawings as S and D respectively. In FIG. 1, the numeral 1 indicates the anode, 2 indicates the cathode, 5 indicates the liquid electrolyte in the gas separation chamber above the cathode 2, 8a indicates the quiescent melt level, S indicates the width of the melt surface, D indicates the depth of the electrolyte in the gas separation chamber above the cathode, L indicates the cathode length, and M indicates the A.C.D. (inter-electrode distance).
Our model studies suggest that, for electrodes inclined at between 7° to 15° to the vertical, conservative or minimum values for S may be attained by applying the empirically derived formula: ##EQU1## where S is in inches, C is the numerical value of the current density in amps/cm 2 , L is the numerical value of the cathode length in inches, and M is the numerical value of the inter-electrode spacing (A.C.D.) in inches. The width of the melt surface and the depth of the liquid electrolyte in the gas separation chamber are preferably not less than twice the inter-electrode spacing, and are preferably not less than four inches. Preferably, S is not greater than D.
There is evidence of the use of vertical electrodes in cells for the production of both aluminium and magnesium, e.g. in U.S. Pat. No. 2,512,157 (Johnson). The Johnson cell, however, is limited to refining impure solid aluminium anodes in chloride electrolyte with deposition of solid aluminium on aluminium cathodes. The lack of provision for adequate gas liberation from between the electrodes would result in low current efficiencies if this cell were adapted for electrowinning of aluminium.
Prior aluminium extraction processes, such as, for example those covered by U.S. Pat. Nos. 2,959,533 and 3,382,166 (de Varda) and 3,352,767 (de Garab et al.), operate with variable A.C.D. and also do not provide adequately for separation of gas. These patents also specify the use of the conventional cryolite-Al 2 O 3 electrolyte, including the consumable carbon anodes which are mandatory in such a system.
In the case of magnesium, the conventional electrolytic cells used for the reduction of magnesium chloride employ vertical anodes in conjunction with high density electrolytes, i.e. electrolytes which are heavier than the molten magnesium produced, and complex cathode designs are thus necessary to collect the molten magnesium at the surface of the melt. Although low-density chloride electrolytes which enable magnesium to be collected on the cell floor are now known, the existing cell designs for use with such electrolytes are quite different from the novel and efficient cell designs proposed in this invention.
U.S. Pat. Nos. 2,468,022 and 2,696,688 (Blue et al.) describe a bipolar electrode magnesium cell of considerable complexity which employs vertical electrodes, but differs from the present invention in a number of important ways. The objectives of the Blue design are to simplify electrolysis chamber construction, to provide improved gas sealing, and to increase the capacity of the cells by providing a larger number of electrodes than in the conventional apparatus. However, these objectives are achieved at considerable cost in complicating the overall cell structure. Thus, a separate two or three compartment feed chamber together with a molten salt distributing system consisting either of a weir chamber mounted above the centre of the cell or a series of distributing channels incorporated into the walls of the cell are required. In addition, a molten salt pump for circulation of the electrolyte is needed.
The Blue cell relies upon forced circulation of melt to remove magnesium from the inter-electrode space in cells which use melt systems of higher density than magnesium. In these cells the molten metal floats to the surface. The cell is said to operate at 0.43 to 0.45 amp/cm 2 , typical of conventional practice. However, current efficiency is said to be about 75 percent, and it is to be noted that no actual operating conditions are described. The low current efficiency is not surprising, because the design is believed to be quite inadequate for securing separation of chlorine from magnesium. Considerable entrapment of gas between the electrodes must occur because not only are the electrodes devoid of any slope, but the inter-electrode gap is also narrow right up to and beyond the surface of the melt.
The cell of Blue et al. does not provide anything approaching the compactness and efficiency of the cell of the present invention, because of the use of low current density and more particularly the lack of provision for adequate chlorine removal, which leads to low current efficiency. Further, the expense of the additional components is likely to nullify to a large extent the claimed advantage of being able to incorporate more electrodes at lower A.C.D. within the electrolysis chamber than in conventional plant.
U.S. Pat. No. 3,396,094 (Sivilotti et al.) illustrates the lengths to which it is necessary to go to improve current efficiency in conventional cells by modifying the method of collecting magnesium at the melt surface. U.S. Pat. No. 3,418,223 (Love) shows the waste of space within a cell resulting from the use of a two-piece cathode for collecting metal at the surface of the melt and also shows the structural complexity of the typical chlorine collection chamber. In both Sivilotti's and Love's cells, separation of chlorine from the melt may be expected to be substantially incomplete because of the lack of adequate free surface for gas separation.
An important feature of one form of our invention is that the cathode surface is inclined at a positive angle, e.g. plus 5° to 30°, to the vertical and faces a parallel or substantially parallel planar anode inclined at a negative angle, e.g. minus 5° to 30°, to the vertical. In other words, it is important in the system of this form of the invention that the working surface of the cathode is arranged to face slightly upwards and the corresponding surface of the anode is arranged to face slightly downwards.
The inter-electrode spacing is preferably less than 2 inches, and desirably between 1.2 and 1.8 inches. As discussed in detail above, study of melt circulation paths leads to the preferred specification of an anode-cathode spacing of about 1.5 inches at current densities of 1.5 amp/cm 2 to 2.0 amp/cm 2 .
Other parameters of importance in the design of the gas separation chamber are the electrolyte depth above the cathode and the width or area of the surface of the electrolyte in the gas separation chamber. It was found that provision of a gas separation chamber extending back 4 to 5 inches from the anode shoulder, across the inter-electrode gap and extending over the top of the cathode structure, was adequate for 12-inch cathodes. For other values of the cell parameters, Formula A referred to above is applicable, within the range of electrode inclinations stated.
The very considerable and somewhat unexpectedly beneficial effect of the electrode arrangement of this invention is demonstrated by comparison of FIGS. 2 and 3 of the accompanying drawings which illustrate the operation of a less favourable and more favourable cell configuration, respectively.
In FIGS. 2 and 3 the numeral 1 indicates the anode having active anode surfaces 1a, 2 indicates the cathodes having active cathode surfaces 2a, 8 indicates the electrolyte, 8a the upper surface of the electrolyte, and 10 the inter-electrode space.
A indicates regions of severe gas formation and B indicates regions of less severe gas formation. In FIG. 3, 9 is the gas separation chamber immediately above the cathode 2.
In FIGS. 2 and 3 the inclination of the active electrode surfaces to the vertical was about 10° and the inter-electrode distance (A.C.D.) was 1.5 inches. In FIG. 2 the current density was 1 amp/cm 2 and in FIG. 3 it was 2 amp/cm 2 . In FIG. 2 the depth of the quiescent electrolyte above the working cathode surface, within the inter-electrode space, was 2 inches; and in FIG. 3 the depth of the quiescent electrolyte above the working cathode surface, within the gas separation chamber, was 4 inches. The width of the upper surface of the electrolyte was 1.5 inches in FIG. 2 and 4 inches in FIG. 3.
It will be seen that retention of gas between the electrodes is very severe under the conditions shown in FIG. 2, even at a moderately low current density of 1 amp/cm 2 . Operation under this condition is found to result in back reaction of about 30 per cent of product, in other words a current efficiency of 70 per cent or less.
FIG. 3 illustrates the effect of electrolysis using a cell design and gas separation chamber constructed in accordance with one form of this invention. Operation with improved gas liberation under the conditions shown in FIG. 3 raised the current efficiency to about 90 percent.
A feature of the form of the invention shown in FIG. 3, as compared with the cell design shown in FIG. 2, is that the width and depth of the gas separation chamber 9 immediately above the working cathode surface 2a are sufficient to ensure (a) that the gas formed in the inter-electrode space 10 during electrolysis is discharged or liberated to a substantial degree from such space 10 into the gas separation chamber 9, and (b) that said gas is liberated to a substantial degree from the electrolyte in said gas separation chamber 9. Preferably the width and/or depth of the gas separation chamber 9 are at least twice the inter-electrode distance (A.C.D.). The above-described cell arrangement substantially reduces the back reaction which would otherwise occur between the gas and the metal in the vicinity of the cathode surface 2a (as indicated in FIG. 2) and thus substantially increases the current efficiency of the cell.
FIG. 4 represents a diagrammatic side elevation of a type of multi-electrode cell designed according to the present invention. Overall dimensions of the cells are preferably as indicated in Table I. 1 designates the graphite anodes with electrolytically active surfaces 1a inclined at a negative angle of about 10° to the vertical, and provided with recesses 4 to form a gas liberation chamber 5 which is preferably proportioned according to Formula A and which affords an adequate rate of gas evolution from the melt surface 8a. 2 designates the cathodes, which for aluminium production are of graphite and for magnesium production may be hollow fabricated steel structures or plain steel sheets, having cathode surfaces 2a which are at a positive angle of about 10° to the vertical and are substantially parallel to the anode surfaces 1a, and 3 represents the refractory-lined steel shell. 8 designates the electrolyte and 6 the electrical current connections for the anodes. Connections for the cathodes 2 are not shown; these may, if desired, be made directly to the shell 3. It will be understood that in any cell configuration the anode(s) may be adjusted in a vertical or substantially vertical direction for setting or re-setting of A.C.D.
FIG. 5 represents a diagrammatic side elevation of a more compact electrode configuration, namely that employing bipolar electrodes. The connotations of the designating numerals used are not the same as in FIG. 4. 1 designates the graphite anode, with working surfaces 1a inclined according to the invention, 2 the bipolar electrodes with working surfaces 2a, which electrodes 2 may be monolithic graphite blocks supported at their ends by the insulating walls of the cell. 3 designates the collector cathodes, which may be of steel in the case of magnesium, but of graphite in the case of aluminium cells, and 3a represents the working surfaces of the cathodes 3. 4 represents the refractory lined outer steel shell and 5 the gas liberation chamber, preferably proportioned according to Formula A. The insulating lower supports 6 for the bipolar electrodes 2 serve as barriers for reducing leakage current.
EXAMPLE 1
A run was carried out in a cell having a single inclined anode, and of the general form shown in FIG. 3. The effective electrode area was approximately 1000 cm 2 . Electrolyte composition was 21% MgCl 2 -75% KCl-4%LiCl, and a total cell current of 400 amps was used, at a temperature of 850°C.
The anode-cathode slope was 9° to the vertical and was within the recommended range for efficient operation. The other parameters were chosen to test some of the less favourable conditions for gas release. The most adverse feature was the use of a melt depth of only 1.5 ins. above the cathode, together with a current density of only 0.36 amp/cm 2 . Under these conditions, considerable back reaction was anticipated to occur because of back circulation of chlorine into the inter-electrode space.
During a run of 61 minutes duration, 404 g of Cl 2 were produced and 543 g of MgCl 2 were consumed.
The current efficiency was 75 percent.
EXAMPLE 2 -- Magnesium
A run was carried out using a single anode cell of the general form shown in FIG. 3 and having an effective electrode area of approximately 1000 cm 2 .
The electrolyte composition was 21% MgCl 2 , 75% K Cl and 4% LiCl.
The anode/cathode slope was 9° to the vertical and was within the recommended range for efficient operation. The cathode length L was 12 inches, S and D were each 4 inches. The operating temperature was 850°C, current density was 0.64 amp/cm 2 and a total cell current of 700 amps was maintained during the 60 minute duration of the run.
801 g of Cl 2 were produced and 1077 g of MgCl 2 were consumed so that current efficiency was 88 percent.
EXAMPLE 3 -- Magnesium
A second run was carried out in the same cell as in Example 2 using an electrolyte containing 22% MgCl 2 , 29% KCl, and 50% LiCl. Operating conditions were chosen to demonstrate one of the optimum combinations of parameters attainable in a cell model.
Current density was increased to 1.5 amp/cm 2 and the cell was operated with a melt depth of 4 in. over the cathode at a temperature of 850°. L was 12 inches and S and D were each 4 inches. During the 45 minute run a steady cell current of 1650 amps was used.
1477 g of Cl 2 were recovered, and 1981 g of MgCl 2 were consumed so that the current efficiency was 90 percent.
EXAMPLE 4 -- Aluminium
A run was carried out in the cell of the general form shown in FIG. 3 using a melt composition averaging 10% Al Cl 3 , 45% NaCl, 45% KCl. The dimensions of the cell were as stated in Example 3. The temperature was 730°C. During the run of 1 hour the cell current was 1400 amps.
1857 g of Cl 2 were recovered and 2323 g of Al Cl 3 were consumed.
The average current efficiency was 89 percent.