Description:
BACKGROUND OF THE INVENTION
Alkali metal halates, such as sodium chlorate, may be prepared electrolytically. In the electrolytic preparation of alkali metal halates, alkali metal halide is fed to an electrolytic cell. For example, in the case of a chlorate cell, alkali metal chloride is fed to the cell. Hydrogen is evolved at the cathode and alkali metal hydroxide is produced adjacent to the cathode. Chlorine and hydroxyl ion come into contact within the electrolyte chamber and react according to equation (i),
Cl2 + 2OH-➝ (Cl--+1 O)-1 + Cl-1 + H2 O (i)
thereby forming hypochlorite ion. The hypochlorite ion, in which the chlorine has a valence of +1, may be self-oxidized to a chlorite ion, in which the chlorine has a valence of +3, and a chloride ion, in which the chlorine has a valence of -1, according to reaction (ii).
(Cl+1 O)-1 + (Cl+1 O)-1 ➝ (Cl+3 O2)-1 + Cl-1 (ii)
The chlorite ion, in turn, is oxidized by hypochlorite ion to chlorate ion in which the chlorine has a valence of +5, as shown in reaction (iii).
(Cl+3 O2)-1 + (Cl+1 O) ➝ (Cl+5 O3)-1 + Cl-1 (iii)
The starting point of the electrolytic alkali metal halate process is the alkali metal halide, e.g., sodium chloride, in which the halogen has a valence of -1. The halogen in the alkali metal halate has a valence of +5. Therefore, the valence change necessary for the production of alkali metal halate is from -1 to +5, a total of +6. In this way, 6 Faradays are required for the production of 1 equivalent of the alkali metal halate.
In the electrolysis of an acidic solution of an alkali metal halide, a hypohalite solution is first produced containing little free hypohalous acid. However, in the presence of a mineral acid such as chromic acid or sulfuric acid, the concentration of hypohalous acid is increased, and the oxidation of the hypohalous acid by hypohalite ions produces halate ion, halogen, and hydrogen. Hydrogen ions then react to form more hypohalous ion and the process continues with the formation of halate ion in all parts of the electrolyte. Side reactions, e.g., the evolution of oxygen at the cathode, and the reaction of nascent hydrogen with oxygen containing ions, may be reduced by the addition of chromate ion, i.e., sodium chromate into the electrolyte thereby favoring the evolution of halite ion.
The chemical formation of halate ion takes place throughout the entire cell, and in fact throughout the system wherever halite ion and hypohalite ion are present.
In the operation of sodium chlorate cells of the prior art, the combination of thick electrodes and low current densities, e.g., less than about 100 amperes per square foot, provided a cell operating temperature of about 50° to about 65°C. In such cells solid sodium chloride had to be substantially continuously added to the cell.
In a batch chlorate cell operation, after the cell liquor is recovered from the cell it is clarified, e.g., by filtration, and then fed to an evaporator for concentration. Afterward, separation occurs and crystallized sodium chloride is recovered. The cell liquor may then be returned to saturating means for adjustment to desired brine strength and returned to the cell. In a continuous chlorate cell process, without evaporation or removal of sodium chloride, the cell liquor is cooled to crystallize the sodium chlorate then returned to the cell.
SUMMARY OF THE INVENTION
According to this invention, bipolar halate cells are provided having compact electrolysis volumes, but permitting the use of large cell bodies, i.e., cell bodies characterized by large electrolyte volumes. In this way, a small volume is utilized for electrolysis while a large volume is provided for the chemical formation of halate ion. Because of the increased electrolyte temperature of the electrolyte, due to the higher current densities obtained with metal electrodes, the solubility of alkali metal halates in cell liquor is increased. The large cell volume relative to electrode volume provides a longer cell residence time. The combination of higher temperature, higher halate solubility, and longer residence time provides higher concentrations of halate in the cell liquor. The longer residence times allows more of the halate to be formed by chemical reaction rather than by electrolysis, thereby providing a higher current efficiency. The higher temperature allows a brine feed to be utilized rather than solid salt feed.
According to this invention, a bipolar alkali metal halate cell is provided having a plurality of individual bipolar units electrically in series. Each bipolar unit has a number of individual conductor elements. An individual conductor element contains a metal anode mounted on one side and a metal cathode mounted on the opposite side. Individual insulators correspond to each pair of individual conductors and are interposed between a pair of individual conductors. In this way, an individual insulator is interposed between a pair of adjacent, individual conductors, and an individual conductor is interposed between a pair of adjacent, individual insulators. Each individual bipolar unit further includes compressive means, imposing a compressive force upon a combination of alternating insulators and conductors. In this way a rigid bipolar unit is provided.
The bipolar units are arrayed in bipolar configuration with the anodes of one bipolar unit interleaved between a pair of cathodes of a subsequent bipolar unit. The anodes of a bipolar unit and cathodes of the subsequent bipolar unit are separated from each other by the insulators, referred to above, of both of the bipolar units. In the same way, the cathodes of a bipolar unit are interleaved between a pair of anodes of a prior bipolar unit, with the cathodes of the bipolar unit, and the anodes of the prior bipolar unit being separated from each other by the insulating means of the pair of bipolar units.
DETAILED DESCRIPTION
The invention may be understood by reference to the appended Figures. In the Figures:
FIG. 1 is a partial cutaway perspective view of the electrolytic cell of this invention.
FIG. 2 is a perspective, partially-exploded view of a segment of a pair of adjacent bipolar units of the electrolytic cell of this invention.
FIG. 3 is a plan view of three bipolar units of the electrolytic cell of this invention. FIG. 4 is a side elevation of a bipolar unit of the electrolytic cell of this invention.
The bipolar electrolyzer 1 of this invention contains a plurality of bipolar units 21 through 24 in series with the subsequent and adjacent bipolar units in the electrolyzer, thereby defining a plurality of adjacent individual bipolar cells 11 through 14. Bipolar configuration may be understood by considering the current flow. Electrical current travels from an anode 51 of one cell 11 attached to the first bipolar unit 21 of that cell 11 to the cathode 61 of the cell. The cathode 61 is attached to the second bipolar unit 22 of the cell 11. The current then passes from the cathode 61 through the conducting means 30 of the bipolar unit 22 to the anode 51 connected to the bipolar unit 22 which is in turn the anode 51 of the next adjacent cell 12 in the electrolyzer 1.
The anodes 51 of the bipolar unit 22 are interleaved between the cathodes 61 of the next adjacent bipolar unit 23. The cathodes 61 of the bipolar unit 23 are interleaved between the anodes 51 of the immediately preceding bipolar unit 22. Direct short circuits between the anodes 51 and cathodes 61 of adjacent bipolar units 21 and 22 are prevented by the insulating means 40 as will be more fully described hereinafter.
An individual bipolar unit 21 contains a plurality of individual conducting means 30. The individual conducting means 30 are interposed between the cathodes 61 of one cell 11 on one side of the bipolar unit 21 and the anodes 51 of the next adjacent cell 12 on the opposite side of the bipolar unit 21. Current travels from the cathode 61 of the prior cell 11 through the conducting means 30 of the bipolar unit 21 to the anode 51 of the next adjacent cell 12. The individual conducting means 30 includes an electroconductive, first metal member 32 having an anode 51 connected to one side thereof, an alkali resistant, electroconductive, second metal member 34 having cathodes 61 connected to the opposite side thereof, and a third metal member 36 between, and mechanically and electrically connected to first 32 and second 34 metal members.
The first metal member 32, i.e., the acid resistant, electroconductive metal member having the anode 51 connected thereto, is fabricated of a material that is resistant to anodic products while retaining its electroconductivity. Most commonly, the acid resistant metal member is fabricated of a valve metal. The valve metals are those metals which form an oxide film when exposed to acidic media or to electroconductive media under anodic conditions. The valve metals include titanium, zirconium, hafnium, vanadium, columbium, tantalum, and tungsten. Titanium, tantalum, or tungsten, are the most commonly used valve metal for electrolytic cell structural members because of their lower cost and ready availability. Titanium is the preferred material for this service because of its lowest cost relative to the other valve metals.
The first metal member or acid resistant, electroconductive metal member 32 is shown in rectangular form. This is because of the ready availability of rectangular stock, the conformity to insulating means 40 as will be described more fully hereinafter, and ease of rigidly connecting the anodes 51 substantially parallel to each other to the metal member. However, it should be understood that the first metal member may be cylindrical or of other shape.
On the opposite side of the conductor 30 is an alkali resistant, electroconductive metal member 34 having cathode fingers 61 connected thereto. Preferably, the alkali resistant, electroconductive, second metal member 34 is fabricated of a material that is resistant to cathode products such as hydroxyl ion while retaining its electroconductivity. Such materials include iron, steel, cobalt, nickel, and the like. Most commonly, iron or steel is used.
While the second metal member 34 is shown rectangular in form, it is to be understood that it may be of cylindrical or other shape. However, a rectangular form is preferred because of the availability of rectangular stock, the conformity to the insulating means 40 as will be described more fully hereinafter, and the ease of welding the cathodes 61 thereto so as to provide substantially parallel, spaced cathode fingers 61.
A third metal member 36 is interposed between, and mechanically and electrically connected to said first metal member 32 and said second metal member 34. The third metal member 36 is typically fabricated of a material that is electroconductive, and substantially impermeable to the flow of hydrogen. Such materials include copper, aluminum, and lead. Most frequently copper is used. The third member 36 provides electrical conductivity between the cathodes 61 of one cell 11 and the anodes 51 of the next adjacent cell 12. Additionally, the copper member prevents the flow of nascent hydrogen from the cathode 61 through the conducting means 40 to the anode 51 of the next adjacent cell.
The copper member 36 may be bonded to the first metal member or acid resistant metal member 32 by welding, friction welding, soldering, bolting, or the like. The copper member 36 may also be bonded to the alkali resistant or second metal member 34 by welding, friction welding, soldering, bolting or the like.
The copper member 36 is shown in rectangular form because of the ready availability of rectangular stock. However, it is to be understood that the copper member 36 may be of cylindrical stock.
Electrodes 51 and 61 are mounted on the opposite surfaces of the conductor 30. Anodes 51 are connected to one side of the first metal member 32. The anodes 51 are substantially parallel to each other and extend from the first metal member 32. The anodes 51 themselves are fabricated of an electroconductive, corrosion resistant metal. Most commonly, the metal will be a valve metal as described hereinbefore, with titanium being the preferred valve metal. The anodes may be in the form of a sheet or plate, or perforate sheet or a foraminous material such as expanded metal mesh.
The anodes 51 are coated with an electroconductive material, having low chlorine overvoltage. Typical materials useful for coating metal anodes for use in electrolytic cells include the platinum group metals, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Alternatively, the coating compound may be an oxide of a platinum group metal such as ruthenium dioxide, rhodium trioxide, palladium dioxide, osmium dioxide, iridium trioxide, or platinum dioxide. Alternatively, the coating compound may be an oxygen containing compound of a platinum group metal such as calcium ruthenate, calcium rhodate, calcium ruthenite, calcium rhodite, the delafossites such as platinum cobaltate or palladium cobaltate, or a pyrochlore such as bismuth rathenate, or bismuth rhodate. Alternatively, the coating material on the surface of the anode may be lead dioxide or other non-precious metal containing oxygen compounds.
The cathodes 61 are connected to the opposite side of the second metal member 34. The cathodes 61 are fabricated of an alkali resistant, hydroxyl ion resistant, electroconductive metal. The cathodes 61 may be fabricated of iron, steel, cobalt, nickel, iron, manganese, or the like. Most commonly, they are fabricated or iron, or steel, because of the ready availability thereof. The cathodes may be in the form of a sheet, plate, perforate sheet or plate, or foraminous or expanded metal mesh. Most commonly, they are fabricated of iron, or steel, because of the ready availability thereof. The cathodes may be in the form of a sheet, plate, perforate sheet or plate, or foraminous or expanded metal mesh. Most commonly they have an open area from about 35 to about 85 percent and preferably from about 65 to about 75 percent.
As shown with particularity in FIGS. 2 and 3, insulating means, i.e., insulators 40 surround the individual conducting means, i.e., the conductors 30 providing separation between adjacent individual conductors 30.
The insulators 40 correspond to the individual conductors 30 and are complimentary in shape to the individual conductors 30 so as to totally cover and fit flush against the copper portion 36. In this way the insulators 40 bear against the copper members 36 and provide a tight fit therebetween, preventing contact of the copper members 36 by the electrolyte.
The insulators 40 are interposed between a pair of individual conductors 30 and are arranged sequentially in an individual bipolar unit in such a way that an insulator 40 is interposed between a pair of adjacent individual conductors 30 and an individual conductor 30 is interposed between a pair of adjacent individual insulators 40.
The insulators 40 include means for maintaining an interelectrode gap between the pair of electrodes 51 adjacent thereto and the pair of electrodes 61 of a subsequent bipolar unit. The insulators also include means for maintaining an inter-electrode gap between the pair of electrodes of opposite charge adjacent thereto 61, and a pair of electrodes 51 of the prior bipolar unit. The inter-electrode gap may be provided by means within the insulators such as a non-conductive structure for engaging the electrodes of the prior bipolar unit, i.e., anodes 51, parallel to and spaced from the oppositely charged electrodes, i.e., the cathodes 61, of the bipolar unit. The insulators 40 also include structure for engaging the electrodes of the subsequent bipolar unit, i.e., cathodes 61, and maintaining them in a spaced relationship, i.e., parallel to and spaced from the oppositely charged electrodes 31, i.e., the anodes, of the bipolar unit. The structural means for accomplishing this may include spaced slot means, grooves, notches, or channels within the insulator. Alternatively, the means for maintaining inter-electrode may include spaced wedges, extended frames, an extended edge, or fin means, such as the extended edge 46 shown in FIGS. 2, 3, and 4.
The individual insulating means include compressible, electrolyte resistant electrically non-conductive, i.e., electrically insulative, means 42 on facing surfaces corresponding to facing surfaces of adjacent conducting means 30. The compressible means may be rubber, polyethylene, Kynar, Teflon, or the like.
Interposed between a pair of the compressible, insulative means 42 is a substantially incompressible, electrolyte resistant, electrically non-conductive, electrically insulating means 44. The substantially incompressible means 44 may be H frames, channel frames, or other shapes. The substantially incompressible electrolyte resistant means 44 include the means 46 for maintaining alignment of electrodes of adjacent bipolar units as shown with particularity in FIGS. 2 and 3.
The individual bipolar units include compressive means 70 providing rigid bipolar structural units. As shown in FIGS. 2 and 3, the compressive means include an electrically insulative bolt means 70 extending through the individual insulating means 40 and the individual conducting means 30 of the individual bipolar unit. The electrically insulative bolt means 70 includes a core 71 of a structural material of high tensile strength, e.g., iron or steel or other structural metal, and may include a sheathing or coating thereon 73 of an electrically non-conductive material. According to one exemplification, the compressive means include iron or steel rod member 71 and a surface 73 of Kynar, Teflon, or the like. At the opposite extreme ends of the compressive means, are bearing surface means 75 which are substantially co-extensive with and bearing upon an external pair of insulating means 46 at the extreme ends of the bipolar unit, and nuts 77.
The individual bipolar units 21 through 24 are assembled into a plurality of individual cells 11 through 14 within a cell body 1.
The bipolar units 21 through 24 may be mounted on base structures 81 through 84 within the cell body 1. This gives electrolyte volume under the electrodes, thereby allowing alkali metal halate formation under the bipolar units 21 through 24 and above the bipolar units 21 through 24.
The bipolar units are shown generally at FIG. 1 and with specific detail in FIGS. 2, 3, and 4. As there shown, the bipolar units are arrayed in series with subsequent and adjacent bipolar units of the electrolytic cell.
The cell body 1 can be rubber-lined metal such as ethylenepropylene-diene lined steel, neoprene lined steel, or the like. Additionally, the cell body 1 can be a concrete body.
The cell body 1 is closed at the top and includes means for feeding brine to the cell and recovering the alkali metal chlorate and hydrogen gas therefrom.
Under normal operating conditions, with a sodium chloride feed, the cell liquor contains from about 650 to about 750 grams per liter of sodium chlorate, from about 60 to about 125 grams per liter of sodium chloride, approximately 2 grams per liter of sodium dichromate added to improve the electrolytical efficiency of the cell, and trace amounts of sodium hypochlorites. In the operation of the cell, the current density is from about 200 to about 600 amperes per square foot. A residence time within the cell of from about 40 to about 250 milliliters per ampere is provided and preferably from about 65 to about 200 milliliters per ampere. The pH of the cell liquor within the cell is from about pH 5.6 to about pH 6.9 and preferably from about pH 6 to about pH 6.8. Under these conditions the temperature of the electrolyte is from about 50°C. to about 100°C., frequently in excess of 80°C. and as high as 95°C. or 98°C. or even 100°C.
In the operation of the bipolar electrolytic cell, feed may either be parallel feed, i.e., a plurality of individual feeds substantially corresponding to each of the individual cells, or the feed may be series feed, wherein the brine is fed at one end of the cell and the alkali metal chlorate is recovered at the opposite end of the cell. Generally, series feed is preferred, as the feed to the first cell is low in hypochlorite ion concentration, thereby providing a high degree of chemical formation of chlorate ion and a high current efficiency.
While the cell has been described with reference to the production of sodium chlorate from sodium chloride, it is to be understood that the cell as herein described above may also be used for the production of sodium bromate from sodium bromide brine, potassium chlorate from potassium chloride brine, and potassium bromate from potassium bromide brine. Although the invention has been described with reference to particular specific details and certain preferred exemplifications, it is not intended to thereby limit the scope of this invention except insofar as the details are recited in the appended claims.