DE1265147B - Process for the production of dithionite - Google Patents

Description

Process for the production of dithionite From the German patent specification 666 184 and the Swiss patent specification 193 915 it is known that dithionite can be produced by sending an electrical direct current through an electrolytic cell according to the following scheme: Cathode compartment anode compartment Ka- catholyte: porous anolyte: water- aqueous solution of salt or water Sulphite solution des Bisulfite wall desired Solution of cation des Dithionite The catholyte is kept in circulation within the cell. From the "Zeitschrift für Elektrochemie", Vol. 36 (1930), pp. 839 to 845, it is also known to use a mercury cathode in the production of dithionite. It has now been found that when a selectively permeable diaphragm is used as a porous partition in a cell for the electrolytic production of dithionite, the technical progress is achieved that a lower potential difference is achieved compared to a non-selectively permeable partition.

The use of electrical energy in the present process for the production of dithionite is therefore greater when the known selectively impermeable Partition is replaced by a selectively permeable partition.

The invention thus relates to a process for the production of dithionite by sending a direct electrical current through an electrolytic cell according to the following scheme: Cathode compartment anode compartment Mercury catholyte: porous anolyte: water silver-aqueous Zwirige salt ode catho- sulphite- solution des de e bisulfite wall you want Solution of cation des Dithionite wherein a dithionite is formed in the catholyte and the constituents of the catholyte are constantly kept in circulation in the cathode compartment of the cell, the catholyte is constantly removed from the cathode compartment and treated in a corresponding manner, and from this the solid dithionite formed is deposited, which is characterized by this is that a porous partition is used, which is selectively permeable to the dithionite cation to be formed and consists of a strongly acidic cation exchange material.

A suitable intermediate wall material consists, for example, of sulfonated polystyrene type of cation exchange resin. By constantly walking around the constituents of the catholyte in the cathode compartment of the cell and constant removal of the catholyte from this it is possible to determine the composition of the catholyte at any one Point of the cell to be kept constant at all times. This seems of particular importance to be, namely by the dissolved concentration of the catholyte cation essentially is kept constant and not more than about 20 ° / o below the concentration decreases at which the procedure is performed.

In a preferred embodiment of the invention, the after the deposition of the solid dithionite mother liquor obtained in the cathode compartment of the Cell supplied again. To this end, the concentration is the same before or shortly after the liquid is fed back into the cathode compartment, to the desired height set again.

For example, the sulfite and bisulfite concentration is thereby filled up that sulfur dioxide is introduced. This can be done inside the cell take place, but this treatment is more conveniently carried out in a separate vessel. The cation concentration (for example sodium ion concentration) can thereby be maintained that the Mother liquor directly a salt des Cation (e.g. sodium chloride) is added. Preferably, however, substantially succeed all added cations in the catholyte from the anode compartment through the porous Partition wall.

The catholyte in the cell is preferably kept moving, for example by means of a corresponding stirring device or baffle walls or both means. The cell can operate at any temperature at which the dithionite product to be produced is stable. In the case of sodium dithionite, the Temperature should be between freezing and about 60 "C, and when the cell is at Operating above about 20 "C, the catholyte should be after being removed from the cell has been cooled to, for example, 20 "C or below, in order to ensure solubility of sodium dithionite. In one embodiment of the invention the cell is operated at 20 to 400 C, and the catholyte is cooled to 0 to 20 ° C, before the sodium dithionite is deposited. It is particularly useful to use the catholyte inside and outside of the cell at a temperature of about 20 "C.

By the method forming the subject of the invention Dithionite produced with a high current yield. Still, as a result of the electrical Losses that are difficult or impossible to avoid completely, the total energy yield of the electrolysis cell is less than the current output, and the resulting waste of electrical energy leads to an increase in temperature the cell. The circulation of the catholyte brings about a certain cooling of the cell. Additional cooling can be done by removing the mercury from the cathode is circulated or by circulating a cooling liquid which with the bottom of the cell near the mercury layer, or both is in touch.

The catholyte preferably contains salts of the cation to be formed of the dithionite in addition to the sulfite and bisulfite, because of the presence such salts reduce the solubility of the dithionite. For the purpose of manufacture of sodium dithionite, for example, the catholyte is saturated with sodium chloride. During the electrolysis, chlorine is formed as a by-product in the anode compartment, this is deducted and should of course be deducted from the catholyte components be kept away.

The pH of the catholyte is preferably within 4.5 to 6.0, for example between 5.0 and 5.5. This pH can be adjusted by external buffering agents be maintained, for example by phosphates, phthalates, and acetates Citrate, however, is preferred to have the buffering effect of the sulfite and bisulfite solution even to use this by controlling the pH, and in practice maintain the pH by increasing the rate of addition of sulfur dioxide is regulated accordingly.

The dithionite is separated from the catholyte liquid preferably through a filter or a centrifuge. Usually contains the mother liquor still dissolved dithionite, but this is essentially replaced by a second Treatment in the electrolytic cell is not significantly influenced and therefore does not work lost. Sodium dithionite is precipitated at a temperature below about 60 "C and collected in the form of the dihydrate Na2S2O4 .2 H2O. This Substance can be known in Way to be drained.

The dithionite product should at any stage of the process, where it is in solution or moist or still contains water of crystallization, handled in an oxygen-free atmosphere.

After dewatering and drying it can of course in air be handled.

In the present process, the current density is for the maximum yield of sodium dithionite per unit of electrical energy consumed about 200 milliamperes per square centimeter of cathode surface. When the current density is increased, it arises in the cathode space in a larger amount of hydrogen as a by-product. Under industrial Conditions it may be beneficial to increase the current density beyond the point in which the maximum power output is achieved, because the economy, which due to the increased output per cell, the lower current yield is achieved more than makes up for it.

The required space velocity of the flowing through the cell Catholyte depends on the current flowing through the cell, and a suitable one Space velocity is within the range 1 to 10, for example 2.5 up to 3 hours-1mp.-l, This corresponds to an hourly volume space velocity of, for example, 1 cm3 of catholyte per cubic centimeter of cathode space per hour per ampere.

The yield of dithionite formed is higher with a higher current concentration, with which the procedure is carried out. If at temperatures within the range from 20 to 30 C is used, the current concentration is at least 10 amperes per liter of total catholyte volume and particularly expediently at least 30 amperes per liter. At higher temperatures, even higher current concentrations should be can be used, but the yields are acceptable at low temperatures at lower current concentrations. The practical upper limit is determined by the requirement set to keep the current density within the specified range.

Since, as already mentioned, the current density is not increased too much should be the required space velocity and current concentration best obtained by having the depth of the catholyte, i.e. H. the distance between the porous partition and the mercury cathode, is kept low.

A suitable distance is up to a few centimeters, for example 1 cm and preferably 0.2 to 1 cm.

To maintain the current concentration, the volume of the The catholyte system located outside the cell is kept as small as possible will.

According to the invention, an electrolytic cell is also used proposed the method described, which is characterized in that it contains a mercury cathode, a porous partition that contains at least one Wall of a cathode and an anode space forms, the cathode space with an inlet and an outlet pipe is provided and the distance between the porous The intermediate wall and the mercury cathode is sufficiently small to allow that an hourly space velocity of the catholyte volume through the cathode compartment of at least 1 ml per hour per ampere of electrical current per milliliter of cathode space is obtained.

The entire apparatus for carrying out such processes is therefore in place from such a cell in Connection with facilities, through which the catholyte is brought into circulation through the cathode compartment, facilities for Separation of the desired products from the catholytes, facilities for replenishing the mother liquor from the separation and recycling devices the mother liquor in the cathode compartment. The apparatus is preferably also with Provide stirring devices for the catholyte in the cell.

The cell is preferably arranged horizontally and the porous The intermediate wall and the mercury cathode are essentially parallel to one another.

Suitable materials for the partition wall are already indicated above.

The invention is explained in more detail in the following examples.

Example 1 The electrolytic cell consisted of a short vertical Glass cylinder of 8.2 cm internal diameter, the bottom of the cylinder with mercury was filled, which formed the cathode. The aqueous catholyte space with a volume of 50 cm3 was located above the mercury at a height of about 1 cm, and the anode compartments, which consisted of three porous shells covered by a rubber seal and which contained saturated brine and graphite anodes, immersed in the catholyte.

The cathode compartment had two inlet openings for gases and one Provided inlet and outlet for the catholyte. Through a special inlet pipe a small nitrogen overpressure was maintained to prevent air oxidation of the Avoid formed sodium dithionite. Gaseous sulfur dioxide was through the other gas inlet tube is introduced directly into the catholyte. The nitrogen kicked through the catholyte outlet tube, which consisted of a vertical tube, wherein the nitrogen carried away the catholyte with it. The catholyte outlet tube led to one of two sintered glass tubes that can be switched on as required, the outlet tubes of which become one Sodium chloride saturation vessel led, whereupon the catholyte returned to the cathode compartment became.

The catholyte circulation with a capacity of about 150 cm3 was filled with catholyte liquid, which at the beginning of the procedure in liter 170 g sodium bisulfite (NaHSO3), 130 g sodium sulfite (Na2SO3) and 200 g sodium chloride (NaCl).

The catholyte was at a rate of 0.7 1 per hour in Circulation set. Using a paddle stirrer, the catholyte and the mercury cathode were touched. Sulfur dioxide was introduced into the electrolytic cell with a speed which was previously determined and which was so great that a pH 5.5 was maintained. A direct current of 5 Ampere passed, and the temperature of the cell was held at 20 "C. After one Treatment time of about 1 hour, the sodium dithionite was precipitated at intervals collected over 10 minutes and dissolved in 500 cm3 of distilled water. By titration with ammoniacal cupric sulfate solution it was found that 2.06 g of sodium dithionite (Na2S2O4) were included, resulting in a current efficiency of 7601o is equivalent to. A second sample made within 10 minutes of another 1 hour treatment time, contained 2.54 g of dithionite, what one Current yield of 940 / o corresponds.

Example 2 In this example, the electrolysis conditions were same as in example 1 with the exception that the temperature of the water bath, in which the cell was immersed, was 40 "C and that the catholyte after leaving of the cathode compartment was collected in a vessel cooled to about 0 ° C. At this Temperature, the sodium dithionite was essentially not in solution, and in this experiment the mother liquor was not returned.

The sodium dithionite sludge, which every 17 minutes Work collected was titrated with ammoniacal cupric sulfate solution. The amount of generated sodium dithionite was 4.4 g, which corresponds to a current efficiency of 96.0 per cent.

Example 3 The electrolytic cell described in Example 1 was modified in that instead of the three porous pots a cylinder made of polymethyl methacrylate with an inner diameter of 5 cm was used, the lower end of which through a 0.03 mm thick disk made of a cation exchange membrane from the sulfonated Polystyrene type of a cation exchange resin ("Permaplex" ()) was sealed The amount of circulating catholyte liquid in this modified cell was 200 cm3.

A current of 2 amps was passed through this modified cell, and the cell temperature was kept at 5 "C. After a treatment time of The catholyte consisting of solid and liquid was drawn off for 84.5 minutes and analyzed in the manner described in Example 1. The total amount of education Sodium dithionite (Na2S2O) was 7.56 g, which corresponds to an average current efficiency of 830 / o corresponded to the entire duration of treatment.

Claims (2)

Claims: 1. Process for the production of dithionite by sending an electrical direct current through an electrolytic cell following the scheme: Cathode compartment l anode compartment Mercury catholyte: aqueous sulfite bi-porous intermediate anolyte: aqueous salt solution anode cathode sulfite solution wall of the desired cation of dithionite a dithionite is formed in the catholyte and the constituents of the catholyte are constantly kept in circulation in the cathode compartment of the cell, the catholyte is constantly removed from the cathode compartment and treated in a corresponding manner in order to deposit the solid dithionite formed therefrom, thereby gekening nz shows that a porous partition is used which is selectively permeable to the dithionite cation to be formed and consists of a strongly acidic cation exchange material. 2. Electrolytic cell for performing the method according to claim 1, which has a mercury cathode, a cathode compartment and an anode compartment, characterized in that the distance between the porous partition and the mercury cathode is so small that it has a volume space velocity through the channel method room of at least 1 ml per hour per ampere of electrical current per milliliter of cathode space allowed. Publications considered: German Patent No. 666 184; Swiss Patent No. 193 915; Journal of Electrochemistry, Vol. 36 (1930), pp. 839 to 845.