BE568676A - - Google Patents

Description

       

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   The present invention relates to the. production of potassium permanganate, and particularly, to an improved, high-yield process of the continuous type of production of potassium permanganate by the electrolytic oxidation of potassium manganate.



   Among the known processes for the production of potassium permanganate by the electrolytic oxidation of potassium manganate, there is a separate batch process in which a solution of potassium manganate is stirred or flowed by gravity in parallel currents through batteries cells with separate cathodes and anodes. Another method is to pass a gravity flow solution of potassium manganate in series through a number of neighboring dipole cells.

   The potassium manganate feed solution is prepared by dissolving potassium manganate in aqueous potassium hydroxide to form a concentrated solution containing in the order of 100-225 grams per liter of potassium manganate. is introduced into a cell and subjected to electrolysis until such time as it is terminally oxidized, or, in the case of oxidation in a number of cells in series, the solution is partially oxidized , the potassium permanganate produced is crystallized, and electrolysis is continued by a second passage through the cells. This process is repeated until the oxidation is complete. Potassium permanganate is separated between the passes to prevent crystallization in the cells.



   The known processes suffer from the disadvantage of the low purity of the crystals of the permanganate produced, which is especially due to the presence of endowed salts of potassium permanganate and potassium manganate, necessitating recrystallization operations and the separation processes that They imply that difficulties are raised by crystallization in cells, cell and conversion yields are often poor and power consumption is often high. The construction of electrolytic cells and their operation require installations of considerable dimensions and significant investments, large spaces, a high cost of operation and labor.



   The objects of the present invention are: to provide an improved process for the production of potassium permanganate which overcomes the drawbacks encountered in the past; to provide a new and improved process which can be performed. continuously and consistently and reliably deliver a high quality product; to produce crystals of high purity potassium permanganate directly, without double salts, while significantly reducing the importance of finishing and separation operations; to provide a process in which the oxidation and electrolysis have a high yield; to provide an oxidation process carried out with controlled flow of electrolyte in a closed cell;

   to provide a process in which the electrolysis conditions are adjusted according to the production of oxygen in the cell; to provide for this purpose an electrolytic cell and an improved electrode, particularly suited to the process; to provide a very efficient dipole electrode for the cell, ensuring high current densities at the cathode and low current densities at the anode.



   These and other objects, the advantages and the possibilities of the present invention will become more apparent '' 'from the description and from the drawings.

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 appended drawings in which the same elements are represented by the same references and in which
Fig. 1 is a diagram of a preferred general process, describing a mode of use of the new cell in relation to the new process carried out therein;
Fig. 2 is a perspective view of a preferred cell or battery of cells embodying the present invention;
Fig. 3 is an enlarged fragmentary perspective and sectional view of the cell;

   
Fig. 4 is another enlarged fragmentary sectional view showing the assembly and construction of the electrodes to form an individual cell of the battery;
Fig. 5 is an enlarged bottom plan view of an electrode, taken along line 5-5 of FIG. 4.



   Fig. 6 is an enlarged top plan view of an end electrode, taken along line 6-6 of FIG. 4;
Fig. 7 is another enlarged fragmentary plan view of a cathode member of the electrode, corresponding to a view taken along line 5-5 of FIG. 4;
Fig. 8 is a side elevation of the element of FIG. 7;
Fig. 9 is an enlarged cross-sectional view of a separator, taken along line 9-9 of FIG. 5;
Fig. 10 is an enlarged cross-sectional view showing the construction of one of the dividers, taken along line 10-10 of FIG. 5.



   Fig. 11 is an enlarged, fragmentary plan view of a top or end electrode taken along line 11-11 of FIG. 3; and
Fig. 12 is a diagram expressing the efficiency of the anode as a function of the oxygen proportion of the gases produced in the cell.



   The novel method and apparatus described herein relate especially to Figs. 2 to 12 of the drawings. Fig. 1 shows the preferred use in the general process of an Applicant's patent of the same date entitled: "Production of potassium permanganate".



  The new process which is the subject of this patent consists in preparing a solution of potassium manganate in aqueous potassium hydroxide, in mixing or diluting the solution with the solution produced, in oxidizing by electrolysis the resulting solution and crystallizing the resulting solution to form crystals of potassium permanganate and a mother liquor which is said above-mentioned product solution. The process can be carried out continuously in an extremely advantageous manner. In the present specification, by "potassium manganate" is meant K2MnO4 in which the manganese has valency 6.



   One of the results of the new process is that a concentrated solution of potassium manganate can be prepared from the outset, which is extremely desirable in the manufacture both from the point of view of the solubility of potassium manganate in solutions. aqueous potassium hydroxide only to maintain solution volumes within practical limits for handling and processing. At the same time, it has been found, according to the present invention, that the powering of the electrolytic cell is most advantageously done by means of dilute solution of potassium manganate. The formation of double salts is avoided, and the purity of the permanganate is greatly improved.



   Dilution of the manganate solution is accomplished by mixing it with mother liquors or other similar materials which are products of 1 '.

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 electrolysis. The purpose of this mixing is also to recycle the potassium manganate which remains after the oxidation. Attention should also be drawn to the very important fact that part of the mother liquors is used to prepare the concentrated manganate solution, which ensures a substantial poncentra- tion of potassium permanganate therein, which serves to avoid hydrolysis of concentrated potassium manganate. At the same time, potassium hydroxide is continuously recycled and reused, part of the mother liquor being withdrawn from the circulation to separate the potassium hydroxide formed during the process.



   The novel general process is therefore preferably a cyclic process of electrolytic oxidation of potassium manganate, of crystallization of the oxidation product to form potassium permanganate crystals and a mother liquor, of removing part of the solution. mother liquor to separate the potassium hydroxide produced therefrom, adding potassium manganate and water to the remainder of the mother liquor and recycling the resulting solution to the electrolytic oxidation
The invention which forms the subject of the present specification relates in particular to the novel electrolytic oxidation process and the novel cell.

   Important features of the new process are, among others, the oxidation in a closed cell, the flow of electrolyte co-circulating with the produced gases, and the adjustment of the electrolysis conditions according to the strength. oxygenation in the cell. The feed solution is preferably a dilute solution of potassium manganate. The cell is made up of a number of individual closed dipole cells which the electrolyte passes through in series.



   The electrolytic oxidation of the potassium manganate solution containing potassium permanganate is carried out so as to obtain an amount of potassium permanganate which is within the limits of the solubility in the solution or electrolyte after the oxidation. Crystallization of the latter solution results in crystals of high purity potassium permanganate, and the mother liquor which remains is reused in the process with the important advantages noted above.



   The new cell includes new electrodes and an advantageous combination and arrangement of elements. The complete cell consists of a number of neighboring dipole electrolytic cells suitable for the application of a series voltage and the series flow of an electrolyte, the dipole cells having electrodes which consist of of a conductive metal sheet forming an anode, of conductive metal elements electrically connected to the sheet and projecting outwardly from spaced points distributed over one of the faces of the sheet and of an insulating material filling the spaces formed between the elements whose outer ends are exposed and form a cathode.



   In Fig. 1 of the drawings, the process begins: by preparing a solution of potassium manganate in aqueous potassium hydroxide = 1 in a leach vessel 1. Fresh potassium manganate is fed to the process, and manganate of recovered and recycled potassium is added to it. The manganate is leached or dissolved in an aqueous solution of potassium hydroxide which consists of mother liquors of the process, containing potassium hydroxide, potassium manganate and potassium permanganate. To dissolve the manganate, the wash water (arrow F) is used, including the permanganate wash water.

   The wash waters replace the water extracted from the cyclic process, and reduce the potassium hydroxide concentration of the mother liquors to the level required for high cell efficiency.



   The leaching solution is driven by a pump 2 from the container
1 to a filter 3, to remove insoluble matter therefrom. The filtrate passes into a second mixing vessel 4 in which it is diluted or mixed with a

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 additional mother liquor produced in the process. In continuous operation, a storage tank, not shown, can be established between the first and second mixing vessels 1 and 40. The quantities of mother liquor introduced into the first and second mixing vessels 1 and 4 are withdrawn from a crystallizer 5 in the vicinity of the upper part of the solution which it contains.



  The materials introduced into the second mixing vessel 4 are heated with live steam (arrow F2), but it is obviously possible to use other heating methods. The solution is then withdrawn and conducted by a pump 6 to an electrolytic cell 7. The solution from the second vessel 4 constitutes the electrolyte of the cell, and passes through the cell in a flow controlled to oxidize the potassium manganate to potassium permanganate. potassium according to the equation
 EMI4.1
 ai-ppr
 EMI4.2
 Cathode Anode K2Mn 4 + H 20 electrol KOH + 1 / 2H2 + ENNO4
The oxidized solution from cell 7 is conducted to a gas separator 8 where the product gases are released from the solution and continuously analyzed to determine the oxygen content.

   The solution is then led to a riser 9, connected to the crystalliser 5, in which it rises at the same time as a large volume of liquor from the crystalliser under the action of a pump 10. The content of the riser is collected. is discharged into a vaporizer 11 which is maintained under reduced pressure, or under vacuum, by a condenser and a vacuum apparatus shown at 12. Evaporation and resulting cooling of the solution takes place in the vaporizer.



   The cooled solution in the vaporizer is over-saturated with potassium permanganate, and it descends into the crystallizer 5 at a point near its bottom. The supersaturation is released in the crystallizer, crystals of potassium permanganate forming on the seeds or crystals present in the crystallizer. This results in the formation in the crystallizer of a quantity of potassium permanganate crystals and a mass of saturated mother liquor.



   Mother liquor is withdrawn from the upper part of the crystalline, as previously described, and potassium permanganate crystals are withdrawn in the form of magma from its lower part, using a pump 13 The potassium permanganate crystals are separated and washed with water in a centrifuge 14, after which they are dried by conventional means, not shown. The mother liquor from the centrifuge is recycled to the crystallizer. Some of the liquor from the crystallizer 5 is also withdrawn and conducted to an evaporator (CE) to separate the potassium hydroxide produced during the electrolytic oxidation and to remove the various salts and impurities, by conventional methods.



   Separated and by-produced materials are reused wherever possible. Thus, the washing water of the potassium permanganate is used to prepare the leach solution from the first mixing vessel 1. By evaporation of liquor, potassium hydroxide is recovered which can be used to prepare potassium hydroxide. potassium manganate. Potassium manganate and potassium permanganate are separated during evaporation, and they can be taken to the first mixing vessel.
The apparatus shown schematically in FIG. read classic, except for cell 7, described below.

   The crystallizer 5 illustrated and used in the method exemplified herein is a Struters Wells suspension vessel with a diameter of 3.05 m and a capacity of about 26,495 liters. The pump 10 connected to the mpntante column 9 starting from the cris tallisoir has a capacity of 11,355 liters per minute. The vaporizer 11 is a Struthers Wells vaporizer with a diameter of about 2.60 meters.



   The new process is preferably and very advantageously carried out

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 using the novel electrolytic cell or battery of cells 7 shown in Figures 2-10. The construction and operation of the cell and the electrolytic oxidation process are the subject of the present invention.



   Fig. 2 shows a complete cell 7 consisting of a battery or column of dipole electrodes 15 arranged side by side and assembled to form a number of individual closed cells. It will be seen that any two neighboring electrodes form a cell. The battery of cells 7, in the illustrated embodiment, is also divided into three sections of cells 16a, 16b and 16c. The feed solution is fed to the battery of cells 7 in series parallel, that is to say that the feed solution passes in series through the cells of each section of cells and that the three sections of cells are supplied in parallel.

   In the particular embodiment shown, it will be found that the separation of the cell bank 7 into three sections of twenty individual cells each is preferable, because the resistance to flow becomes important for more than twenty cells in series.
The feed solution is therefore supplied through a main pipe or conduit 17 to three parallel groups of feed conduits 18a, 18b and 18c of the sections, which enter the first cell of each of the twenty sections. The oxidized solution leaves the twentieth or last cell of each section through parallel groups of conduits 19a, 19b and 19c, from where it is conducted to the gas separator 8 via an exhaust conduit 20.

   Between each supply and output duct and its respective collector, there is a section of about 1.22 meters of glass pipe, for example 18aa or 19aa, for electrical isolation purposes.



   The electrodes 15 are assembled into a gas-tight complex by tie rods 21 or other similar elements. The upper and lower end electrodes 22 and 23 are insulated by insulating sheets 24 and 25, preferably based on asbestos. Bus bars 26 and 27 are electrically connected to the respective end electrodes 22. and 23, adjacent to one end thereof as well as to a direct current source, preferably about 140-170 volts, depending on the current desired. To the individual cells and to the electrodes 15 is applied a voltage in series, so that the potential difference in each of the sixty cells is one sixtieth of the total voltage.

   In this connection, it is evident that the cell sizes of the corresponding voltage and total applied current are determined on the basis of practical considerations and from the power available. The electrolytic oxidation process performed in the cell is fundamentally only a function of the current density at the electrodes.



   Les'fig 4 to 8 show in detail the construction of the electrodes 15 when they are mounted. The electrodes are dipolar and each of them constitutes both an anode 28 and a cathode 29, except the upper, - and lower electrodes 22 and 23 which necessarily perform only one function, Item: neighboring electrodes - together close a cell , the anode of one and the cathode of the other. The electrolyte of the solution to be oxidized and under oxidation flows between the anode and the cathode of neighboring electrodes.



   For the oxidation, the current density at the cathode 29 must be much higher than that at the anode 28. The current density must be high at the cathode to prevent a build-up from occurring there. reduction of potassium permanganate. The current density must be low at the anode for the anode to have a high yield and to achieve satisfactory oxidation of potassium manganate to potassium permanganate and low oxygen formation. In the past, the anode was made of bare metal sheet, and the cathode was a smaller sheet metal, or covered with a perforated insulating material, such as the transits to reduce the effective area of the cathode and increase - ter the current density.

   In addition, diaphragms were arranged between the electrodes.

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 -brodes. However, these perforated materials were unsatisfactory because their openings were quickly blocked with solids. The electrode 15 shown very satisfactorily provides the necessary current density, and the difficulties caused by the deposition of solids are very substantially reduced. The surface of cathode 29 is so constructed. to avoid this fouling. The anode should only be cleaned after periods of one to six weeks to remove solids.



   Electrode 15 is constructed of materials resistant to alkaline manganate solutions at the temperatures used under the conditions of electrolysis. Thus, a backing sheet 30 of 11 gauge hot rolled steel can be used. The backing sheet of the upper and lower electrodes 22 and 23 protrudes from the ends of the cell to form a flange on each of them.
31 allowing the assembly of the bus bars 26 and 27.



   The surface of the anode 28 of the electrode has one or more wire screens 32, preferably of monel metal, which are electrically connected to each other, and to the backing sheet 30 by spot welding, or a analogously, at the edges and at a number of points distributed over the surface of the anode.



   On the surface of the cathode 29 of the electrode, a sheet 33 of 22 gauge cold rolled steel is electrically connected to the support sheet 30, for example by welding. The sheet 33 is stamped or cut to form elements. metallic conductive elements 34 projecting perpendicularly outward, partially separate and integral with the sheet or connected to it, as shown in Figs. 4, 5, 7 and 8, and leaving corresponding perforations or openings in the sheet. The spaces between the conductive elements 34 on the surface of the cathode are filled with an insulating or non-conductive material
36 which may be a synthetic organic resin or other material offering the required resistance to the conditions of electrolysis.

   Polystyrene is preferably and very well used. Another thermoplastic resin which can be used is a polymer of tetrafluoroethylene.



   To construct the electrode, the anode screens 32 and the cathode sheet 33 are connected to the support sheet 30 and the spaces between the conductive elements 34 are filled with polystyrene molding powder which is heated and plasticized to. completely fill and seal the spaces. Then, the polystyrene is milled to expose the steel ends or ends 37 of the conductive elements, having the appearance shown in FIG. 80
In this way, a small cathode area is exposed, consisting of the ends of the conductive elements evenly distributed at spaced points over the entire surface of the cathode 29. In the representative embodiment, 4% of the surface of the cathode. cathode are metal corresponding to the exposed edges of the elements and the remainder is polystyrene.

   The reverse side of electrode 15, which constitutes the anode, preferably has a substantially increased conductive surface, because of the screens 32 and the large additional surface which they develop. One or more, and preferably two, screens 32 are used, and more can be used if desired. The screens perform another useful function in that they cause turbulence or agitation of the trolyte flowing through the cell which very favorably affects the efficiency of the anode conversion.



   Figs. 5 and 6 show one of the individual cells, the first cell or lower end cell of the cell bank 7. In the cell arrangement, the electrode of FIG. 5 can be visualized as being turned upside down on the surface of the electrode of FIG. 6 of how to make a book. In the construction of the cells, sealing or closure spacers 39 are disposed at the edge of the electrodes, and flow separators 39 are disposed on the surfaces of the electrodes.

   To mount the cell the spacers and separators are driven on the cathode surface 20,

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 which) they are welded and to which they adhere by plastic welds, for example, by applying polystyrene in a solvent such as toluene to the welds, and by evaporation of the solvent.

   Prior to assembly, a layer of heavy tar material 40 is spread over the exposed edges of spacers 38 and dividers 39, to provide a tight seal when the plates are stacked and bolted together or assembled from one another. way, .We will see in Fig. 6 that the screens 32 are located on the surface of the backing sheet 30 by means of a jig or the like, leaving uncovered or exposed areas 41 on the surface of the backing sheet which correspond to the locations of ; spacers'38 and separators 39, to allow assembly of the parts of the cell.



   Fig. 3 shows how the supply ducts 18a-C and the exhaust ducts 19a - c are connected to the cells. They pass through the spacers 38 and communicate with the interior of the cells. The supply and discharge ducts are made of a non-conductive material, preferably of tetrafluoroethylene polymer. Fig. 9 shows the slight taper of the separators 39, the purpose of which is to ensure greater tolerance to the manufactured screens 32 in the mounting of the cell.



   The more specific description given here and in the example refers particularly to a cell 7 which includes electrodes 15 having ends 121.9 cm wide and 243.8 cm sides, including the intercalates. - laries 38, but extensions 31 excluded. This surface is reduced by the spacers 38, the width of which is 50.8 mm and which have a depth of 31.7 mm. The dividers 39 have a width of 25.4 mm at the base and 12.7 mm at the top and a depth of 31.7 mm. The distance between the cathode of one electrode and the anode of the neighboring electrode is 22.2 mm, surface to surface, allowing a flow of liquid of approximately the same thickness or depth.



  There are provided two screens of superimposed anode wire 32, 8 meshes, of 0.047 gauge monel metal. The ends of the cathode elements 37 are 6.3 mm long, and the elements 34 protrude 4.8 mm from the surface of the cathode sheet 33. There are about 5000 cathode elements. The conductive surface of the cathode is 930 cm2, and that of the anode is 140,000 square centimeters (ratio 1: 150),
In use, the cells are arranged so that the anodes 28 lie below the cathodes 29, but in principle their arrangement could be reversed. The flow of the solution to be oxidized or electrolyte is shown in FIG. 5 referring to the separators.

   Figs. 5 and 11 illustrate, respectively, the supply of the battery of cells 7 and the discharge of this battery. The liquid enters through the supply conduits 18a - c above the first electrode 15 of each cell section 16a - c, into the channel defined by the end separator 39a and the spacer 38. The channel is isolated by an asbestos-based strip 29a arranged on the surface of the cathode 29. The liquid passes through the cell in a tortuous current around the separators and leaves the cell through an opening 42 made in the electrode at the end of the cell. end opposite the entry point.

   The liquid enters through the opening into the next neighboring cell, which is above the previous cell, into the serial flow, in the embodiment shown The arrangement of the separators 29, and the The corresponding flow of liquid into the next neighboring cell is a mirror image of those shown, such that opening 42 communicates with a channel which is a reproduction of that shown as formed by the end separator 39a and tab 38, which is on the opposite end in the next cell.

   The liquid in the next cell therefore flows in the opposite direction and leaves it through another opening similar to the opening 42 of the first cell, at the opposite end of the next electrode. liquid flows in series in a back and forth motion from one cell to the next until it passes through it. last or twentieth cell in each section of cells. The liquid then leaves the battery of cells 7 at the end where the supply takes place, through the evacuation conduits 19a - c, which form

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 protrusion in the end of the cell in the area corresponding to the discharge opening 42 which is in the preceding cells (see Fig. 11).



   The cell and the flow are designed to establish a co-flow of liquid and gases, so that the cell operates in forced circulation without hindrance caused by the gases. The gases exit smoothly and continuously together with the oxidized solution through the exhaust line 20, and the mixture is led to the gas separator 8. The gases are released and continuously analyzed to establish their content. in oxygen, on the basis of which the operation is adjusted, as will be seen below. To achieve this co-current flow, the bottom of the cell, as shown, forms an angle 9 with horizontal, on Fig 1. This view is a representation of the narrow or end side of the cell.

   Thus, the spacer 38a of each electrode adjacent to the discharge opening 42 is located at the upper edge and the opposite spacer 38b at the lower edge. The liquid flows in each cell from the zone adjacent to the lower edge towards the area adjacent to the upper edge and this flow is accompanied by a rise in gas. As each cell is fed at the high edge 38a, a bypass opening 43 is provided in the end separator 39a at its upper end (see Fig. 10). The opening bypasses or releases the gas that is not conducted by the liquid in that area. To establish the co-current flow, it is also possible to mount the cell so that the electrodes extend at different angles openly.



   Although the apparatus and arrangement of the elements illustrated are preferred, it is evident that many modifications can be made thereto without departing from the scope of the present invention.



   To carry out the process, the concentration of potassium hydroxide in the first mixing vessel, or leachate 1, must be high enough to avoid significant hydrolysis of the potassium manganate. This concentration should not be too high, because it can, later in the process, be detrimental to the yield of the cells and to the purity of the crystals.



  It is preferable that the concentration of potassium hydroxide is about 70 to 150 grams per liter, and more preferably 80 to 120 grams per liter.



   The concentration of potassium manganate should not be too high because the hydrolysis increases with the formation of permanganate and manganese dioxide. With low manganate concentrations, excessive amounts of liquid must be handled. It is preferable that the potassium manganate concentration is about 100 to 200 grams per liter, and better still, 120 to 180 grams per liter.



   It is also preferred that there is as much potassium permanganate as possible in the leach liquid to prevent hydrolysis of the concentrated manganate. Thus, a solution containing about 100 grams per liter of potassium hydroxide and about 150 grams per liter of potassium manganate is preferably saturated with potassium permanganate, which is about 50 grams per liter.



   The leach solution is prepared to about 50-6000, filtered, and then introduced into the second mixing vessel 4. In this vessel, the leach solution is diluted with several volumes of mother liquor from crystallizer 5, the leach solution being diluted with several volumes of mother liquor from crystallizer 5. propprtions depending on the amounts of material present in the two liquids. The mixture is heated, for example with live steam, as necessary, to bring it to a temperature which is preferably about 55 to 75 C.



   The solution thus prepared for the electrolytic oxidation preferably contains 80 to 180, and more than 120 to 150 grams per liter of potassium hydroxide. The concentration of potassium manganate is preferably about 35 to 80 grams per liter, and more preferably 50 to 60 grams per liter. The concentration of potassium permanganate is preferably less than about 35 grams per

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 The proportions of manganate and of permanganate are chosen so that, under working conditions, the amount of potassium permanganate produced in cell 7 is within its limits of solubility in the solution or electrolyte after oxidation.

   In this case, there is no problem of crystallization in the cell.
The solution is oxidized at a temperature of about 55-80 C. The initial solution enters the cell at a preferred temperature of about 55-75 C, and the temperature in the cell rises by about 3-5 C. Higher temperatures should be avoided because they reduce the efficiency of the cell.



   The process is carried out so as to obtain an oxidized solution preferably containing about 30 to 70 grams per liter of potassium permanganate, and more preferably 45 to 55 grams per liter. The resulting concentration of potassium manganate is preferably about 15 to 50 grams per liter, and better still 20 to 30 grams per liter, 15 to 20 grams substantially constituting the end point of the electro'Lytic oxidation. It is preferable to avoid an increase in the amount of potassium manganate remaining so as not to prejudice the purity of the crystals produced and to minimize the amount of sub-salts to be treated. By "end point" is meant the concentration of potassium manganate at which the rate of manganate oxidation falls rapidly.

   The concentration of potassium hydroxide increases in proportion to the formation of the permanganate, with 1 mole of potassium hydroxide being produced per mole of permanganate produced. The proportions of the materials are, after oxidation, in a consistent relation to the initial proportions, such that potassium permanganate is within the limits of its solubility in solution.



   In this connection, the production of a solution of a higher concentration of potassium permanganate tends to reduce the efficiency of the anode.



  The production of potassium permanganate is further controlled by the amount present in the starting solution, described above. In addition, it is preferable that the electrolyte is superheated by at least about 5 ° C to avoid possible crystallization of the permanganate in the cell which would be due to local supersaturation.



   The feed solution is supplied to the described cell 7 at a rate of about 151 to 284 liters per minute, preferably about 170 to 246 liters per minute. The flow is sufficient to create turbulence around anode 28, preferably about 10.2 to 15.2 cm per second on the anode.



  The flow rate is also correlated with the rate of oxidation, determined by the current input, and with the temperature of the solution, which determines its capacity with respect to the permanganate. Thus, for example, for a current of 1200 amperes and a corresponding production of 5.44 kg of permanganate per minute, the solution being at 62 ° C., a flow rate of approximately 227 liters per minute is chosen.



   The current density at the cathode is preferably equal to 50 to 400 times the current density at the anode, and better still 100 to 200. To ensure these relationships between the current densities, the conductive surfaces of the cathode and anode are in inverse ratio, i.e. 1:50 to 400. The current density at the cathode for the described embodiment is preferably 0.4 to 2.0 amps per cm2, and better from 0.6 to 1.5 amps per cm2. The anode current density is preferably 0.003 to 0.014 amps per cm2, and more preferably 0.005 to 0.009 amps per cm2.

   The exposed cathode area represented by the ends 37 of the conductive member 34, and the exposed anode area represented by the screens 32 and the adjoining area of the backing sheet 30 are determined so as to ensure the densities of. adequate current. The potential difference in each cell is determined by the current density and resistance of the cell. In the embodiment shown, the resistance is low and the voltage of the individual cell is in the range of about 2.3 to 2.8 volts.



   After initiation of the operation, the working conditions of the cell

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Iule and electrolysis conditions are set.!;! basically, according to the present invention, as a function of the production of oxygen in the cell, which production is determined by the analysis of the exit gases. To do this, it is first necessary to determine the efficiency of the cathode under given conditions of current density and working temperature. This is done in a known manner by determining the production of hydrogen and comparing it with the theoretical production. At the same time, the optimum current density is determined at the cahode.



   For a given current density and working temperature, the efficiency of the cathode remains relatively constant when the supply conditions, namely the flow rate of the solution or its composition, vary. Is calculated- for a given cathode yield, the oxygen content of the outlet gases (H2 and O2) corresponding to the various yields of the anode? An anode efficiency versus oxygen content is plotted for the cathode efficiency at which the cell will be put into service, as shown in Fig. 12 of the drawings. This diagram plots the percent oxygen content on the ordinate and the anode percent yield on the abscissa. The two curves represent cathode efficiencies of 90% and 80%, respectively.



   In practice, the oxygen content of the outlet gases is measured and the efficiency of the anode is determined on this basis by referring to the diagram.



  The invention is carried out with an anode efficiency in the neighborhood of 90-92%. The efficiency drops when the electrolyte flow is too slow, the manganate concentration of the feed solution is too low, or when deposits build up on the anode. Therefore, if the oxygen content shows a significantly lower yield, indicative of greater electrolysis of the water, the feed rate is increased to bring more manganate to the oxidation and restore the expected yield. of the anode. Alternatively, the same result is achieved by increasing the manganate concentration in the feed solution.



   Still alternatively, the current density can be reduced, by lowering the voltage, to restore the expected efficiency of the anode. When the change is not substantial, the variation in cathode efficiency can be neglected, especially at high levels of cathode efficiency.



  Otherwise, a different curve is used on the diagram, which curve corresponds to the different yield resulting from the cathode. Thus, the operation of the cell is regulated in a simple and safe way according to the formation of oxygen in the cell. When the buildup of deposits on the anode reaches the point where the operation loses its effectiveness, the cells are cleaned.



   The solution of the oxidation product is carried from the gas separator ** 8 to the riser 9, connected to the crystallizer 5 as described above. The pump 10 circulates the mother liquor and the solution of fresh product with a flow rate of 110355 liters per minute, the vaporizer is under an absolute pressure of about 35 mm of shit, and the temperature prevailing in the vaporizer. 11 and in crystallizer 5 is about 38 C, in the embodiment described. However, these conditions can be varied. Thus, a higher vacuum can be used with correspondingly increased evaporation and cooling in the vaporizer.

   More potassium permanganate then crystallized in the crystallizer, and the mother liquors contained less potassium permanganate.



   The mother liquors are taken to mixing vessels 1 and 4, as described, the proportions being adjusted so as to obtain the compositions described above. A certain quantity of liquid is also withdrawn from the crystallizer to separate therefrom the potassium hydroxide formed during the process and to remove impurities therefrom. The crystallized potassium permanganate produced is withdrawn from the bottom of the crystallizer by pump 13 as a magma of about 30% solids. the rest of the treatment is carried out as described above.

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   The following example illustrates the execution of the process according to the present invention, but it is obvious that the invention is not limited to the particular installation, proportions, conditions and processes described.



  EXAMPLE .-
A leach solution is prepared in the first mixing vessel 1 from fresh potassium manganate of about 82-90% purity and from recovered salts, from the crystallizer mother ligator. and washing water. The solution, prepared at 50-60 C, has the following composition:
 EMI11.1
 
<tb> 100 <SEP> grams <SEP> per <SEP> liter <SEP> KOH
<tb>
<tb>
<tb>
<tb> 150 <SEP> grams <SEP> per <SEP> liter <SEP> K2Mn04
<tb>
<tb>
<tb>
<tb> 50 <SEP> grams <SEP> per <SEP> liter <SEP> KMn04
<tb>
 53 liters per minute of the solution are filtered through filter 3 to remove impurities and insoluble matters therefrom, and then taken to the second mixing vessel 4.



   In the second mixing vessel, the leaching solution is mixed continuously in the proportion of 1 part by volume to about 3 parts by volume of mother liquor from crystallizer 5, the mother liquor being at 380C and having the following composition:
 EMI11.2
 
<tb> 135 <SEP> grams <SEP> per <SEP> liter <SEP> KOH
<tb>
<tb>
<tb> 25 <SEP> grams <SEP> per <SEP> liter <SEP> K2MnO4
<tb>
<tb>
<tb>
<tb>
<tb> 25 <SEP> grams <SEP> per <SEP> liter <SEP> KMnO <SEP> 4 <SEP>
<tb>
 Live steam is introduced into the second container until the temperature reaches 65 C.

   The resulting solution for the oxidation has the following composition
 EMI11.3
 
<tb> 120 <SEP> grams <SEP> per <SEP> liter <SEP> KOH
<tb>
<tb>
<tb>
<tb> 53 <SEP> grams <SEP> per <SEP> liter <SEP> K2MnO4
<tb>
<tb>
<tb>
<tb>
<tb> 30 <SEP> grams <SEP> per <SEP> liter <SEP> KMnO4
<tb>
 
The solution is continuously conveyed from the second vessel 4 by the pump 6 to the cell, 74-at a flow rate of 189 liters per minute, in three parallel streams which pass through the three sections of cells 16a -c. Direct current is supplied to the cell, 155 volts and 1150 amps. The resulting density of the current at the cathode is 1.3 amps per cm2, and at the anode 0.009 amps per cm2. The temperature of the output current of the cell is 68-70 C.



   The oxidized solution flows to the gas separator 8 where the gases are released. The gases are continuously analyzed for oxygen content using a Beckman apparatus and the percentage of oxygen in the gases is continuously recorded. The cell should operate with an anode efficiency of about 90-92% and a cathode efficiency of about 89%, which corresponds to an average oxygen production of about 4-5%, as the shows Fig 12.



  If the oxygen content rises above 6%, the flow rate of the leach solution is increased to about 60-61 liters per minute, and this content usually drops. If it does not fall back, but continues to rise, the cell must be stopped and cleaned, after which its operation is again brought back to normal.



  When the flow rate of the leachate solution is increased as described above; the amount of mother liquor mixed with it in the second vessel 4 is kept constant, as is the feed rate to cell 7. This results in an increased manganate concentration in the feed solution to the cell. cell.

   Operating in this way, the product from the cell has the following average composition:,

 <Desc / Clms Page number 12>

 
 EMI12.1
 
<tb> 128 <SEP> grams <SEP> per <SEP> liter <SEP> KOH
<tb>
<tb>
<tb> 24 <SEP> grams <SEP> per <SEP> liter <SEP> K2MnO4
<tb>
<tb>
<tb> 53 <SEP> grams <SEP> per <SEP> liter <SEP> KMnO4
<tb>
 
The hot product solution is mixed with the sieve solution, which is at 38 ° C, and the mixture is pumped through riser 9 at a flow rate of 11,355 liters per minute. The absolute pressure prevailing in 1 vaporizer is approximately 35 mm Hg, and approximately 13.2 liters per minute of water is vaporized, which cools the solution of this riser from approximately 38.5 C to 37.8 vs.



   The crystallization which takes place in the crystallizer leaves a mother liquor having approximately the following composition:
 EMI12.2
 
<tb> 135 <SEP> grams <SEP> per <SEP> liter <SEP> KOH
<tb>
<tb> 25 <SEP> grams <SEP> per <SEP> liter <SEP> K2MnO4
<tb>
<tb> 25 <SEP> grams <SEP> per <SEP> liter <SEP> KMnO4
<tb>
 
22.7 liters of mother liquor are continuously recycled from the crystallizer to the first vessel 1, and 136 liters per minute of mother liquor are continuously recycled to the second vessel 4. 30.3 liters of mother liquor are taken from the crystallizer to the second vessel. Evaporators E. 125 grams of K2MnO4 per liter of the leach solution is oxidized to 99 grams of KMnO4 which is separated in centrifuge 14. (output 5.2 kg per minute).



    @
The potassium permanganate crystals * in the centrifuge 14 are washed with water, and the washing water is reused in the preparation of the solution of the leaching vessel 1. The crystals are dried in hot air at 120 ° C. The potassium permanganate thus obtained has a purity of 99.40%. The yield of potassium permanganate, based on the potassium manganate consumed, is about 99% in cell 7. The overall yield of potassium permanganate based on the fresh potassium manganate fed to the process is about. about 98%.



   The cell operates with an overall electrolytic efficiency of 82% (potassium permanganate produced relative to the current required). The energy consumption is low, about 0.55 kw per kg of salable potassium permanganate produced.



   The present invention therefore provides a new and improved process for the production of potassium permanganate which lends itself well to a continuous or semi-continuous production. The process directly gives high quality permanganate in high yields. The yields of electrolysis and conversion are high. The device is small, and very simplified, and can be easily implemented by a small number of personnel. Capital investments, maintenance, and labor costs are very low.



   CLAIMS.



   1.- A process for the electrolytic oxidation of potassium manganate to potassium permanganate, characterized in that a solution of potassium manganate in aqueous potassium hydroxide is passed through a closed electrolytic cell by concurring with the gases produced.



   2. - A process for the electrolytic oxidation of potassium manganate to potassium permanganate, characterized in that a solution of potassium manganate in aqueous potassium hydroxide is passed through an electrolytic cell in controlled flow. , and the conditions of the electrolysis are adjusted according to the production of oxygen in the cell.

** ATTENTION ** end of DESC field can contain start of CLMS **.


    

Claims (1)

3.- Process for the electrolytic oxidation of potassium manganate in <Desc / Clms Page number 13> in potassium permanganate, characterized in that a solution of potassium manganate in aqueous potassium hydroxide is passed in series through a certain number of closed dipole electrolytic cells to which a voltage is applied in series, the solution passing through these cells in forced circulation in co-current with the flow of the gases produced. 4.- A process for the electrolytic oxidation of potassium manganate to potassium permanganate, characterized in that a solution of potassium manganate in aqueous potassium hydroxide in series is passed through a number of electrolytic cells. closed dipoles to which a series voltage is applied, the solution passing through these cells in forced circulation in co-current with the flow, of the gases produced, and the supply conditions are adjusted according to the production of oxygen in the cell. 5.- Process for the electrolytic toxidation of potassium manganate to potassium permanganate, characterized in that a solution of potassium ma ganate in aqueous potassium hydroxide is passed through a closed electrolytic cell in controlled flow. , in co-current with the flow of the gases produced, the solution containing approximately 35 to 80 grams per liter of potassium manganate and the manganate is oxidized therein by electrolysis to permanganate at a temperature of approximately 55 to 80 C to obtain a aqueous solution of potassium hydroxide containing about 15 to 50 grams per liter of potassium manganate and about 30 to 70 grams per liter of potassium permanganate, the proportions of the materials in the solution before and after the oxidation being such that the amount of the resulting potassium permanganate is within the limits of its solubility in the solution after the oxidation. 6.- A process for the electrolytic oxidation of potassium manganate to potassium permanganate, characterized in that a solution of potassium manganate is passed in aqueous potassium hydroxide and !. series by a number of closed dipole electrolytic cells to which a voltage is applied in series, the ratio between the current density at the cathode and the current density at the anode being in the range of 50 to 400: 1 the flow of the current being uniformly distributed over the surface of the solution, the solution passing through the cells in forced circulation in co-current with the flow of the gases produced, 7.- Electrode for a dipole electrolytic cell, characterized in that it comprises a conductive metal sheet forming an anode, conductive metal elements electrically connected to this sheet and projecting outward from spaced points distributed over one of its surfaces and an insulating material filling the spaces between these elements, elements whose outer ends are exposed and form the cathode. 8.- Electrode for dipole electrolytic cell, characterized in that it comprises a conductive metal sheet electrically connected to a screen of conductive wire disposed on one of the surfaces of the sheet, this combination; forming an anode, conductive metal elements electrically connected to this sheet and projecting outwardly from spaced points distributed over the opposite surface and an insulating material filling the spaces between these elements, elements of which the outer ends are exposed and form cathode. 9.- Electrode for dipole electrolytic cell, characterized in that it comprises a conductive metal sheet forming an anode, conductive metal elements electrically connected to uette sheet and projecting outwardly from spaced points distributed over one of its surfaces and polystyrene filling the spaces between these elements, elements whose outer ends are exposed and form the cathode. 10.- Electrode for electrolytic cell, dipolar, characterized in that it comprises a conductive metal sheet electrically connected to a <Desc / Clms Page number 14> screen of conductive wire disposed on one of its surfaces, this combination forming anode, conductive metal elements electrically connected to the plate and protruding outwardly from spaced points distributed over the opposite surface, an insulating material filling the spaces between these elements, the outer ends of these elements being exposed and form the cathode, and separators made of insulating material arranged on one of the surfaces of the electrode to EMI14.1 providing a tortuous path for the liquid gas current flow between the anode and the cathode of neighboring electrodes. II.- Electrolytic cell, characterized in that it comprises a certain number of neighboring dipole electrolytic cells designed for the application of a voltage in series for the flow through them of an electrolyte in series, dipole cells comprising electrodes which comprise a conductive metal sheet forming an anode, conductive metal elements electrically connected to the sheet and protruding outwardly from spaced points distributed over one of its surfaces and an insulating material filling the spaces between the elements, elements whose outer ends are exposed and form a cathode. 12.- Electrode designed to create high current densities at points distributed over a certain surface, characterized in that it comprises a conductive metal sheet, a number of metal elements EMI14.2 conductors protruding outward from spaced points distributed on a d-6 sele âaurfâesteêsré'étarif'paihlérertl..éidde? t.rart: è ert armant dastie 'integral this; cea: c 1. .rae: mt = t = selt e.phi-Wt: e # SJ.: re: m; re esé1êmefttso with the outer ends exposed.