MXPA02009013A - Electrolyte and diaphragm for fused salt electrolysis. - Google Patents

Abstract

An electrolytic cell for the production of chlorine and an alkali metal from a fused chloride electrolyte comprising at least one graphite rod anode, a concentric cylindrical cathode (14) surrounding each anode, a rigid cylindrical diaphragm (10) positioned between said anode and cathode, and self aligning means (15) that align said diaphragm concentric with, and at a predetermined distance from, said anode and cathode.

Description

ELECTROLYTE AND DIAPHRAGM FOR ELECTROLYSIS OF CASTED SALTS FIELD OF THE INVENTION This invention relates to an electrolytic cell for the electrolysis of alkali chloride salts, molten to produce alkali metals such as sodium and lithium.

DESCRIPTION OF THE RELATED ART The electrochemical cells for the electrolysis of molten alkali metal salts are widely used in industry to produce meta! It is alkaline, such as sodium and lithium, which are difficult to reduce to a metallic state. A higher cost of operating these cells df the cost of electricity. Since since 1970, the cost of electric power has risen steeply. Therefore, the development of other efficient energy electrolysis processes has become increasingly important. The electrolytic recovery of sodium metal is commercially carried out via electrolysis of molten, non-aqueous chloride salt. While the subsequent discussion concentrates on the elaboration of sodium, the characteristics related to the design of the cell and mechanical operation REF. 141289 above 650 ° C. The literature describes numerous other ternary mixtures. The choice of mixture depends on such factors as the melting temperature of the mixture, its electrical conductivity, the desired purity of the resulting sodium, and the possible deposition of the salts at various points in the apparatus due to differences in solubility. at the lower temperatures found in some parts of the sodium cell. These factors affect the operability of the cell, how many times the cell must be stopped for repair, current performance and cell productivity, and in general what is referred to in the industry as the "health" of the cell A modern cell Downs it typically contains four graphite carbon rods that serve as anodes. Each anode is surrounded by a concentric steel cylinder that serves as a cathode. In operation, sodium is deposited on the inner surface of the steel cathodes and chlorine gas is released into the graphite anodes. Typically, in a cell with four pairs of electrodes, the chlorine is collected in four axes of the anodes while the sodium is collected in a single compartment that completely covers the four cathodes A hydraulically permeable diaphragm is used to separate the cathode and anode compartments to prevent the reaction and mixing of sodium and chlorine again. It is typically made of steel mesh, and it has a relatively short life of about 2 months because it corrodes and clogs with those of echos. When the diaphragm reveals some larger holes, it must be replaced because the holes lead to the reaction and mixed again of the sodium and chlorine, in turn reducing the current yield and energy efficiency. Diaphragm replacement is an expensive and labor-intensive step. Current diaphragm designs have a number of defects. A defect is that the diaphragms are typically rigidly attached to the sodium :: collection by means of a steel ring bolted to the collar. The connection of the diaphragm to the sodium collector is done through a laborious operation in a specially designed "slit". Following the junction pass, the diaphragm is transported to the cell and lowered into place. Because the bolting design is rigid, and because there are slight mechanical variations from cell to cell, this procedure rarely achieves perfect alignment between the new diaphragm and the electrodes in the cell along the entire length of the cell. cell. He imperfect alignment or partial short circuit between anodes and cathodes, reducing the current in the cell. The improved current efficiency is a major area to save potential energy. While the performance of an electrolytic process could theoretically be over 99%, most commercial sodium salt melt cells operate at relatively low current efficiencies. The Ullmann Encyclopedia, for example, indicates a typical current efficiency of 80 to 90% (p.287). Another important area to save energy is to decrease the volt- age drop across the cell. Typically the voltage drop across the electrolyte-filled space between the cathode and the anode accounts for approximately 40% of the electrical energy required for operate a sodium cell. The reduction of the electrical resistivity of the molten electrolyte could result in significant energy savings for the operation of the cell. However, in order to keep the operation quiet, any new electrolyte composition should not increase the melting temperature of the mixture or the tendency of associated metal salts to separate by precipitation from the solution, and must produce a sodium metal of acceptable purity. Preferably, a new electrolyte composition should also improve the operability and quality of the cell.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides an electrolytic cell for the production of chlorine and an alkali metal from a molten chloride electrolyte having at least one tubular anode with graphite, a concentric cylindrical cathode surrounding each anode, a diaphragm rigid cylindrical placed between the anode and the cathode, and isolated alignment means joining the diaphragm and the anode or cathode to concentrically align the diaphragm when placed in position (ie, the diaphragm is self-aligning). In a preferred diaphragm, the alignment means are sets of insulating rollers, suitably mounted on the outer surface of the diaphragm to join the internal surface of the mat when the diaphragm is inserted in position. In one embodiment, the self-aligning diaphragm has a flotation chamber that causes the diaphragm assembly to float in the electrolyte. In another modality, the Self-aligning diaphragm mechanically stops in position by means of a stop mechanism mounted on the upper part of the diaphragm that joins a structure of the sodium collector assembled from the cathode. The invention also provides the following electrolyte compositions for the production of chlorine and sodium: (a) about 20 to 40% by weight of NaCl, 30 to 50% by weight of BaCl2, 15 to 30% by weight of CaCl: and 0.2 to 13.0 by weight of LiCl, (b) approximately 20 to 40% by weight of NaCl, 5 to 15% by weight of aCl2, 50 to 70% by weight of SrCl2 and 1.0 to 13.0% by weight of LiC-L, and , (c) about 20 to 40% by weight of NaCl, 50 to 80% by weight of SrCl2 and 0.2 to 13.0% by weight of LiCl.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. A and IB are vertical and horizontal cross sections, respectively of a typical Downs cell having four sets of electrodes. FIG. 2 illustrates a modality of the self-aligning diaphragm of this invention.

Figure 3 illustrates a second self-aligning diaphragm mode < ion of this invention.

DETAILED DESCRIPTION OF THE INVENTION This invention provides several substantial improvements to the mechanical and electrolytic properties of an electrolytic cell for the production of molten alkali metal and chlorine gas by the electrolysis of molten chloride salts. While the mechanical and electrolytic improvements are discussed separately, one or more of these improvements can be incorporated into a unique design of an improved electrolysis cell. Although the description is given in terms of electrolyzing spdio chloride, the mechanical improvements of the improved cell can also be used for the electrolysis of lithium and other alkali metals.

Downs Cell Figures 1J and IB, respectively, illustrate vertical and horizontal cross sections of a Down type cell having four sets of electrodes. The cell tier e a cylindrical shirt of steel 1, covered with bricks. Cylindrical anodes of graphite 2 are projected upwards through the lower part of the steel shirt. The patches 3 are steel cylinders having two diametrically opposed steel arms 4 projecting to the outside of the cell jacket to serve as electrical terminals The cylindrical steel screen mesh diaphragms _5 are suspended around the intermediate in the annular space between anodes and cathodes. The annular collector ring 6 collects the molten metal that arises in the molten electrolyte 7 from the cathodes. The exit tube _8 transports the metal collected in the collector ring to the outside of the cell. The gas chamber 9 transports the gaseous anionic products formed by the electrolysis. The elements 5, 6, 8_ and _9 are supported in the cell by means not shown, typically by rigid means such as screws, welding or conventional bolts. Currently, a steel mesh screen is used as a diaphragm to separate the cathode and anode compartments. The diaphragm prevents the reaction and mixing again of the alkaline metal cathodically produced and anodically produced chlorine. The relatively short life of the diaphragm, combined with the method of labor intensive replacement and alignment of the same, is a major cost factor in the operation of the cell Do ns. In addition, such diaphragms They are of limited efficacy, partly because of the alignment deficiencies, with cell grids that typically achieve full flow efficiencies in the range of 80% to 90%.

Auto-align diaphragm The diaphragm designs of the current invention overcome these limitations of the prior art by providing a self-aligning diaphragm. By "self-alignment" it is meant that the diaphragm is aligned by itself concentric with, and at a predetermined distance from the cathode and anode when the diaphragm is inserted in its place. Figures 2 illustrates a self-aligning diaphragm mode provided by this invention. The diaphragm 1_0 is made of sieving or grooved materials as described in the prior art, but has the following characteristics that make it self-aligned.

Instead of a bolted, rigid connection between the diaphragm and the sodium collector 1_1, the diaphragm floats in the electrolyte and rests with the lower part of the sodium collector, separated from it electrically by a number of supports 2 electrically insulators mechanically resistant, such as modified spark plugs, attached to intervals around the upper part of the diaphragm, These insulating supports are clamped so that their lower parts will resist at the cathode 14 when the float of the diaphragm is in its lowest position. Furthermore, attached to the upper part of the diaphragm is a flotation chamber 13, a device similar to a hat containing small purge holes in the upper part. The volume of the flotation chamber is adjusted so that the diaphragm will rest against the sodium collector in normal operation, sustained afloat by the upward flow of chlorine gas collected in the chamber. When the electricity for the cell is reduced or cut completely, the chlorine slowly escapes through the purge holes, causing the diaphragm to float down or submerge it to the point where the insulating supports rest on the upper surface of the cathode. This (movable diaphragm has at least two sets of isolating roller separators 1_5, one near the bottom of the diaphragm and one set higher above the diaphragm, to provide the self-alignment feature. Only the upper assembly is shown. The free space between the roller separators and the cathode will be sufficient to allow the diaphragm assembly to move freely up and down, but not so broadly to allow misalignment that could unnecessarily increase in the path of current flow, which could increase the cell voltage required for operation. In operation, the flotation chamber fills with chlorine gas emitted at the anode, the remaining amount of chlorine is diverted from the flotation chamber and into the collection system. The chlorine in the flotation chamber keeps the entire diaphragm assembly afloat until the top of the isolating sleeves rests against the sodium collector. Therefore, the need for a rigid connection or screw connection to the collector is avoided, eliminating the costly slit operation required for repair and replacement by the conventional design.When the cell run is cut, the emission The chlorine in the chamber is stopped and the chlorine in the flotation chamber escapes through the small purging holes, the chamber gradually fills with molten electrolyte and loses its flotation, causing the diaphragm assembly to slumber until The insulating supports rest on the upper surface of the cathode, this oscillating movement can be achieved by deliberately turning on and off the current of the cell.

Oscillating movement is very useful in the breaking and cutting of calcium dendrites that are frequently formed during cell operation, partial short origins, electric arc formation and loss of current performance. The insulator roller separator assemblies keep the diaphragm centered and short-circuit against the electrodes during this operation. Different media from insulated rolls can be used to self-align the diaphragm, and the media can be mounted on the diaphragm, cathode, anode, or other structural element of the cell. Figure 3 illustrates a second embodiment of the self-aligning diaphragm of this invention. As in the first embodiment, the day: ragma 20 is made of conventional grooved or sieved materials. The diaphragm has a metal part 2_1 rigidly attached to its upper portion containing a number of L-shaped notches, of which the notch 2_2 is shown in the side view. Joined at each notch is a bar, | of which the bar 23 is shown in the front view. These bars are rigidly fastened to the sodium collector, but n are fastened to the diaphragm. The notches and bars are positioned so that the diaphragm assembly can be inserted from below the sodium manifold, with the vertical portion of each notch in line with Each adaptation bar, then move up and rotate (as if you screw a glass jar on its lid) at the end of the trip of the notch. A small upward widening of the notch at its end stops the diaphragm in position within the cath or 24. The clearance between the enlarged notch is closed and the bars are sufficient for a light lateral free movement of the diaphragm. In order for this slightly movable diaphragm to be self-aligning, it has at least two sets of insulating roller spacers 25, one near the bottom of the diaphragm and a set up above the diaphragm to provide the self-aligning feature of this design. . Only the upper set is shown in this Figure. The clearance between the roll spacers and the cathode wall is sufficient to allow the diaphragm to be rotated in position, but not so widely to allow misalignment that could unnecessarily build up in the path for the flow of current. which could increase the cell voltage required for the operation. Different media from insulated rolls can be used to auto-align 1 diaphragm, and the media can be mounted on the diaphragm, cathode, anode, or other structural element of the Ida.

As in the previous modality of the diaphragm, there is no need for a rigid or screwed joint between the diaphragm and the sodium container, thus eliminating the expensive "slit" operation required for repair and replacement by the design. conventional. Insulating supports and insulating roller separators for the above diaphragms can be made of any insulating materials which have mechanical properties and adequate strength at low temperatures and high insolation values in molten electrolyte, such as silicon nitride (Si3N4), alumina (Al203) and other materials known from those in the art. The axes on the rollers can be any rigid material which is suitable for the environment c.e bath, preferably a metal such as steel. While the invention has been described in detail with respect to a preferred embodiment wherein the insulating rolls are employed as the alignment means, it will be appreciated that the equivalent means may be selected to space the diaphragm concentrically with the anode and the cathode. For example, the rigid separation means may be mounted on the inner surface of the cathode. Similarly, different means of the flotation chamber I .. the "health" of the cell Correspondingly, the poor operability cells are referred to as "sick" cells. For the health of a cell, it is important that the electrolyte has a broad ratio of compositions that remain completely fused over a wide range of temperatures. The ability of a substance to promote the free movement of molten electrolyte salts over a range of temperatures is referred to herein as their "flux" capability. Another important character of the electrolyte is its conductivity. The voltage drop across the electrolyte-filled space between the anode and the anode for a typical electrolyte composition NaCl-CaCl 2 -BaCl 2 is almost 3 volts, accounting for approximately 40% of the electrical energy required to start a sodium cell. Other typical electrolytes have similar voltage drops. Any reduction in the electrical resistivity of the molten electrolyte resulted in significant energy savings for the operation of the cell. It is known that the cclloorruorroo dde lliittiioo (: LLÍCICIl): tiíeennee electrical resistivity substantially lower than the ingredients in the typical mixtures above. Previous attempts to use lithium chloride as an electrolyte component were unacceptable, NaCl weight, 48% by weight of BaCl :, 26% by weight of CaCl .. studied the effect of small additions of LiCi to the bath. The addition of LiCl transforms this ternary system into a quaternary system for which no published data is available. These compositions were submitted to tests of thermal analysis (CAD / Heat Differential Analysis Metrics) to determine their melting temperatures, by which proposes the temperature at which all the material melts, The results were as follows Table 1 Compo calcula tion calculated,% by weight Additions NaCl BaCl- CaCl- LiCl Temp. of Fusion, ° C Control (without LiCl 26.0 48.0 26.0 0.0 575, 579 addition of 1% LiCl 25.7 47.5 25.7 1.0 566, 568 addition of 2% LiCl 25.5 47.1 25.5 2.0 563, 564 addition of 5% LiCl 24.7 45.7 24.7 553, 554 addition of 10% LiCl 23.6 43.6 23.6 9.1 514, 499 addition of 20% LiCl 21.7 40.0 21.7 16.7 480, 482 addition of 40% LiCl 18.6 34.2 18.6 28.6 520 The experimental results obtained by this The system shows that the LiCl additions, even in really small, significantly lower the melting temperature of the electrolyte compositions which will improve the operability of the sodium cells. The strongest effect on the melting temperature decrease is between the addition of 0.2% to 10% LiCl. The rise in temperature between 20% and 40% LiCl indicates the presence of a eutectic within this range of composition for this quaternary mixture. An addition range of 0.2 to 15% of LJC1 is preferred for reasons of economy, corresponding to a composition of about 20 to 40% by weight of NaCl; 30 to 50% by weight of BaCl2; 15 to 30% by weight of CaCl ?; and 0.2 to 13.0% by weight of LiCl. A similar series of experiments are directed to the effect of relatively small LiCl additions on the melting temperature of a ternary strontium chloride-based electrolyte for the preparation of sodium (26% by weight of NaCl, 12% by weight of Ba ( fl, 62% by weight of SrCl2.) The addition of LiCl transforms this ternary system into a quaternary system for which no published data is available.The electrolyte compositions were prepared containing 5% by weight and 10% by weight of LiCl added to the previous bath based on estror ció.These compositions are they underwent thermal analysis tests as before to determine their temperature: melting. The results were as follows: Table 2 Calculated calculation,% by weight Additions NaCl BaCl- SrCl- LiCl Temp. of Fusion, ° C Control (without LiCl) 26.0 12.0 62.0 0.0 545 addition of 5% LiCl 24.7 11.4 59.0 4.8 515 addition of 10% LiCl 23.6 10.9 56.4 9.1 462 It was seen from the previous datings that even small additions of LiCl will significantly lower the melting temperature of the strontium bath, and for which substantially the operability of such a bath is enhanced by preventing freezes and similar problems. An addition range of 0.2 to 15% by weight of LiCl is preferred for reasons of economy, corresponding to a composition of about 20 to 40% by weight of NaCl; 5 to 15% by weight of BaCl2; 50 to 70% by weight of SrCl2; and 0.2 to 13.0% by weight of LiCl.

Similar experiments were conducted in the binary system of NaCl and SrCl; The published data shows a eutectic composition of 30% by weight of NaCl and 70% by weight of SrCl2 with a eutectic melt temperature of approximately 570 ° C. The melting temperature rises steeply with small changes in the composition, allowing only 15% broad range of compositions before the melting temperature can exceed a typical cell operating temperature of 600 ° C. With the addition of 11% by weight of LiC ^ the above eutectic composition, the following results were obtained Table 3 Calculated composition,% by weight Additions NaCl SrCl; LiCl Temp. of Fusion, ° C Concrol without LiCl) 30.0 70.0 0.0 570 addition of 11% LiCl 27.0 63.1 9.9 479 The above results show that even small additions of LiC - have a powerful fluxing effect about the binary NaCl / SrCl2 system. That is, the small additions of LiCl in a much wider range of melting temperatures, which improves the operability at the operating temperature of 600 ° C typical of sodium cells. An addition range of 0.2 to 15% by weight of LiCl is preferred for reasons of economy, corresponding to a composition of approximately 20 to 40% by weight of NaCl, 50 to 80% by weight of SrCl2; and 0.2 to 13.0% by weight of LiCl. To determine s: the relatively small percentages of lithium chloride could produce a sodium cell product with acceptable purity, laboratory experiments were designed and conducted to determine the degree of lithium recovered by sodium metal in contact with electrolyte containing Lithium Loride at 600 C under unbalanced conditions ie without agitation). The conditions select approximately several simulated conditions in the electrolytic cell and cover a wide range of exposure times, which vary from the time of a few seconds required for the sodium drops to emerge through the electrolyte bath at several hours when a thick layer of Sodium metal within the collector is in discrete contact with, and floats on, the molten electrolyte. The electrolyte in these experiments contains (by weight) 4.8% of LiCl; 24.7? of NaCl; 24. |% CaCl :; and 45.7- of BaCl. The results of this preliminary study are shown in the Tac Table 4 Recovery of LjLtio by Sodium Metal These tests show that, although there is considerable dispersion in these data, the absolute level of lithium recovered by sodium metal under these conditions is minimal. It is also important to know if the Li co-deposited with Na on the electrode. Such co-deposition could be of D-use and highly undesirable use of Li-containing electrolytes. To estimate the thermodynamic driving force for the co-deposition of Li with Na for small additions of LiCl, the EMF 600 ° C intervals between Na and Ll are calculated for the previous compositions of electrolyte L based on calcium and based on strontium. The higher the EMF interval between the Na is, the lower the Li the nobler the smaller the tendency to co-deposit from the Li For an addition of 5% LiCl to the calcium base electrolyte, the EMF range increases from approximately 0.1 volts rated at standard EMFs between Na and Li at 600 ° C at approximately 0.2 volts. This is a large increase in the EMF range, and it means that at low LiCl concentrations the driving force for Na deposition without Li deposition is a favorable result. Favorable rests were obtained for the strontium base bath. Using the literature data on the electrical conductance for LiCl, NaCl, BaCl2 and CaCl2, it is estimated that the cell voltage change for a bath containing 10% LiCl based on the typical electrolyte based on previous calcium chloride it could be approximately a reduction of 0.5 volts to 0.8 volts, corresponding to approximately 7% to 11% of energy savings, The tests of pla y confirm previous preliminary information. Even in LiCl addition amounts as low as 0.2 to 5% by weight results in a perceptible increase in current yield. In the electrolyte a calcium chloride base, an addition of 0.2 to 3% by weight of LiCl shows approximately 2% higher current efficiency. In addition, a more uniform temperature distribution throughout the cell is noted, a variation of 10 ° C from top to bottom against approximately a variation of 30 ° C without the addition of LiCl, and therefore greater problem-free operation of the cell , that is, very few enhancements, sickness "or" smoking "of the cell and fewer adulterated freezes near the bottom of the cell and in other locations.With the time, this will result in higher average energy yield and fewer requirements of operation and maintenance work Ls say, the addition of LiCl to the typical compositions of sodium electrolyte surprisingly produces better operability of the cell It is noted that in relation to this date the best method known by the applicant to carry out the practice said invention is that which is clear from the present description of the invention.

Claims (1)

1. 4. The electrolytic cell according to any of claims 1 to 3, characterized in that the self-aligning means are the means of flotation. The electrolytic cell according to claim 4, characterized in that the flotation means is a lance chamber. The electrolytic cell according to any of claims 1 to 3, characterized in that the means of self-ation are the means for lateral free movement, liefer: or of the diaphragm, 7. The electrolytic cell according to claim 6, characterized in that the self-aligning means additionally comprise at least two sets of insulating roller spacers, one of which is close to the lower part of the diaphragm. 8. The electrolytic cell according to claim 1, 2 or 3, characterized in that a flotation chamber is present in the upper part of the diaphragm, the flotation chamber causes the diaphragm assembly to float in the electrolyte of 1 cell while the cell is in operation. 9. The electrolytic cell according to claim 4, characterized in that the isolating separators are present in the upper part of the flotation chamber for electrically separating the flotation chamber from a sodium collector placed above the flotation chamber. 10. The trolitic cell according to claim 1, 2 or 3, < : characterized in that the diaphragm has a metal part attached to its upper portion, the metal part has a pLurality of notches, and the electrolytic cell has a sodium collector mounted above the diaphragm and has a number of bars projecting from this They are fixed in the slots to position the diaphragm concentrically with the anode and cathode of the cell when the diaphragm is rotated to stop the bars in the slots. II. An electrolyte composition for the production of chlorine and sodium from molten chlorine electrolytes, characterized in that it consists essentially of about 20 to 40% by weight of NaCl; 30 to 50% by weight of BaCl;; 15 to 30 '% by weight of CC1 ::; and 0.2 to 13- by weight of LiCl 12. An electrolyte composition for the production of chlorine and sodium from molten chlorine electrolytes, characterized in that it consists essentially of approximately 20 to 40% by weight of NaCl; 5 to 15% by weight of BaCl2; 50 to 70% by weight of SrCl ?; and 0.2 to 13.0% by weight of LiCl. 13. An electrolyte composition for the production of chlorine and sodium from molten chlorine electrolytes, characterized in that it consists essentially of about 20 to 40% by weight of NaCl; 50 to 80% by weight of SrCl2; and 0.2 to 13.0% by weight of LiCl