US 2825685 A
Description (OCR text may contain errors)
arch 4 1958 o. SCHACHTER ET AL 5 2 9 PROCESS OF ELECTROLYSIS OF AQUEOUS ELECTROLYTES Filed April 12, 1955 ELECTROLYSIS OF AQUEOUS ELECTROLYTES Application April 12, 1955, Serial No. 500,915 6 Claims. (Cl. 204-128) PROCESS OF This invention relates to the electrolysis of aqueous electrolytes for producing bromine or iodine by processes in which the products obtained at the anode and cathode, respectively, are apt to react with one another chemically if they are allowed to mingle in the aqueous medium.
Electrolytical processes in which none of the reaction products escapes in gaseous form but the reaction products have to be separated from one another in liquid form for the aforesaid reason, are hitherto carried out in cells provided with diaphragms. These cells are relatively costly, and in their operation there arise problems of corrosion and clogging. A diaphragm-less cell, the socalled bell-jar cell, is known for the manufacture of chlorine. In this cell the anolyte is separated from the catholyte by a neutral zone of the electrolyte located below the anode. The location of the neutral zone in the cell is not stable, as owing to the difierence in the velocity of migration of the hydrogen and hydroxyl ions, respectively, the zone tends to be shifted towards the anode. In order to counteract this displacement the fresh brine is supplied to the cell from above into the region of the anode above the neutral zone which is thereby kept down. The chlorine escapes as a gas through an outlet provided above the anode. This known cell cannot be used for the production of bromine or iodine since, being heavier than water, these two elements would, at the moment of their formation, sink down, pass the neutral zone and react with the NaOH of the catholyte to form sodium hypobromite and sodium hypoiodite, respectively.
It is a primary object of the invention to provide a continuous electrolytical process for aqueous electrolytes containing bromine and iodine which can be carried out in a simple apparatus without diaphragm.
The invention has furthermore the object to provide a process for the electrolysis of bromide or iodide solutions containing both cations that form water-soluble hydroxides, and cations Whose hydroxides are insoluble, wherein the whole or a substantial part of the hydroxide formed by the process can be removed from the apparatus as a sediment without interruption of the electrolysis. in particular, the invention provides a continuous electrolytical process for the production of pure magnesium hydroxide from aqueous bromide or iodide solutions in which magnesium is only one of the cations.
The invention consists in a continuous process for the electrolysis of aqueous bromide or iodide solutions substantially without formation of gaseous products of electrolysis at the anode and without separation of the anolyte from the catholyte by a diaphragm, wherein a stream of fresh electrolyte is fed immediately into the electrochemically neutral zone of the aqueous medium between the anolyte and catholyte.
Preferably, the current is made to pass between electrodes disposed mainly in the upper part of an electrolytic cell, both the anolyte and catholyte are withdrawn from the upper part of the cell and the fresh electrolyte 2,825,685 Patented Mar. 4, 1958 "ice is fed into the electrochemically neutral zone below the reach of the electrodes and the anolyte and catholyte drains and is made to flow upwards so as to prevent the free halogen formed by the electrolysis from dropping into and through the neutral zone.
Where the electrolyte consists principally of alkali bromide or iodide, the product obtained at the cathode is alkali hydroxide which is apt to some extent to diffuse back into the neutral zone and further into the anode space, thus diminishing the current efiiciency. It has, therefore, been found to be advantageous to admix the electrolyte with salts of metals whose hydroxides are insoluble or difiicultly soluble in water, such as magnesium or calcium. Magnesium salts happen to be present anyway in many natural brines or residual industri l liquors which come into regard as sources for the production of bromine or iodine, such as sea-water, the water of some salt lakes such as the Dead Sea, or mother liquors remaining from the manufacture of sodium chloride from seawater, the manufacture of carnallite from natural salt deposits or salt lakes, or the like. Where such liquors are subjected to electrolysis by the process according to the invention, the principal products are bromine on the one hand and a highly pure magnesium hydroxide on the other hand. The purity of the magnesium hydroxide thus obtained makes it even worthwhile to add magnesium salt, e. g. chloride, to alkali halide electrolytes with the main object of producing pure magnesium hydroxide.
Calcium hydroxide is more soluble in water than magnesium hydroxide, and the current efficiency is, therefore, lower in the case of precipitation of calcium hyroxide than in the case of magnesium hydroxide.
Where the process according to the invention serves for the preparation of bromine, the rate of inflow of brine can be so regulated that the anolyte is an aqueous bromine solution from which the bromine can be recovered in any suitable way, e. g. by solvent extraction. If the bromine is liberated by secondary reaction of the bromides of the brine with the chlorine set free from the chlorides of the brine by electrolysis, the brine inflow should be so regulated that about 2% of the bromides remain undecomposed, in order to avoid the formation of chlorine-bromine compounds.
The accompanying drawings show diagrammatically in axial section two cells for performing electrolytic processes according to the invention.
Fig. 1 shows a cell of small laboratory dimensions;
Fig. 2 illustrates a cell designed for commercial use.
The cell according to Fig. 1 consists essentially in an upright U-shaped glass vessel 1. The upright tube 2 of the vessel 1 is the anode space, the left-hand tube 3 is the cathode space. The tube 2 and 3 are provided each with overflows 4 which open into drains 5, 6, provided wtih cocks 1', 8, respectively. The upper ends of the drains merge into vents 9. A feeding pipe it) provided with cock 11 opens into the horizontal bottom stretch of the vessel 3. substantially in the center thereof. The plus signs in the anode space, and the minus signs in the cathode space, indicate that the liquid in these spaces is not neutral electrochemically, the pH being clearly below 7 in tube 2, and above 7 in tube 3. The liqr id in the bottom stretch of the glass tube, in which the supply pipe Iii) opens, is electrolytically neutral, i. e. at a pH of 7 or in the vicinity of 7. The approximate boundaries of this neutral zone are indicated by dashed lines.
The anode space contains a graphite anode 12 which is a rod secured in an axially bored stopper 1% which tightly seals the tube 2. A current lead is is connected to the top end of the anode.
A cathode 15, e. g. an iron cylinder, is disposed in the cathode space, being freely suspended from any suitthe receptacle 7 upper end of the tube 7 wire 16.
Fresh electrolyte is supplied continually through pipe '10, and liquid flows out continually through bothipipe's Sand 6. If, for example, sea-water or salt panbrine, is the electrolyte, the anolyte flowing cutthro'ugh drain 5 -is'an aqueous solution of bromine, possibly containing some iodine, While the catholyte flowing cutthrough the drain 6 contains alkali hydroxide. Besidea-magnesiuni hydroxideseparates-in the tube 3 and gradually clogs the latter.
This cellis destined for experimental or demonstration purposes only, and it has been shown here merely because it illustrates the principle of the present invention in an especially simple way.
The electrolytic cell illustrated in Fig. 2 is designed for operation on a' commercial scale. it comprises an the anode space and the anolyte coated with a corrosion-resistant of baked-on phenol-formaldehyde theinner surface of drain, are preferably matter, e. g. a layer resin.
Owing to the fact that fresh electrolyte is fed into a zone below the reach of bothclectrodes and has to flow upwardsin the cell, neither the free halogen formed in the anode space nor :t hemetal hydroxides forming in the cathode space can drop intothe' neutral zone whose position within the -cell thus remains "stable. 'ThisstabilitY of the neutral 'zoneis alsoenhanced by thefact thatiboth the anolyte and catholyte are drained from the-upper part of the cell.
upright cylindrical vessel 17 of, stainless steel 'made in- 'tegral with a jacket 18 which is provided with an inlet 19 and an outlet 2% for a coolant, e. g. water. The inclined bottom 21 of the vessel 17 supports the legs 23 of a cup-shaped receptacle lindrical chamber 24, also or" stainless steel and'co-axial with the receptacle 22 and the vessel 17,, is disposed above 22 and dips into the latter, and an annular passage 25 between the receptacle 22 and chamber 24 makes the interior of the chamber 24 (anode space) communicate with that of the vessel 17 (cathode space). The chamber 24 is secured, e. 'g. welded, to its lid 26 which is clamped between'nuts 28 screwed on bolts 27. The latter are fixed 'to a cover 29 which rests unconnectedly on the common upper edge of the vessel 17 and jacket 18 and has in its middle a sleeve 3% for guiding the chamber 24. Thelatter can be raised and lowered relative to the receptacle 22 by a corresponding adjustment of the nuts 28, whereby the length of the passage 25 can be varied with a view to preventing the liquid contained in'the anode space from mingling with that contained in the cathode space.
The lid 26 is provided with a tubular connection 31 co-axial with the chamber 24. A rod-shaped anode 32, e. g. of graphite, is fixed to a cap 34 which rests on the 31 and is insulated electrically from The anode hangsdown through the tube 31 into the chamber 24 and ends substantially flush with the bottom end of the chamber where it merges preferably with a disc 33 of the same material. A current lea 35 is connected to the cap extension 36 servingras a vent The annular cathode space the wall of vessel 17 contains a cylindrical cathode 37, c. g. of iron, which is fixed in any suitable manner, e. g. suspended from a rod 38 which rises out of the cathode chamber through an opening 39 in the cover 25 and serves also as a current conductor.
A supply and which contains the electrochemically neutral portion of the liquid in the cell. A distribution head 41 may be provided at the downwards bent end of pipe 49.
The cellis provided with three outlets, namely, an 'anolyte overflow 42 comprising a vertical stretch within the anode space where its upper end defines the highlevel of the'anolyte, which is located in the upper part of the cell, while the lower part of the anolyte drain passes outwards through the receptacle 22, the cathode space and the jacketya catholyte overfiow43 passing from the cathode space outwards through the jacket sub stantially at the level of the upper end of the anolyte overflow; and a sediment drain 44- connected to the deepest region of the cathode space. Control valves, which have not been shown, will be provided for the. supply pipe and all the drains.
The surfaces of the cell liable to the latter.
for the anode space. between chamber 24 and become corroded by contact with the products of the electrolysis, especially '22 of'stainless steel. A cyliter per hour.
34. The tube 3i has a lateral ethylene .dibromide The invention is illustrated by the following examples to which it is, of course,.not amples, a cell of the kind illustrated in Fig. 2 may be used, but the dimensions of the cell and the input'of electrolyte and output of 'anolyte and catholyte are' described on a small laboratory scale. For .thefigures'given in the'examples, a cell with a capacity-05250 ml. of total Enlargement of :the cell :to dimenthe commercial performanceof the a correspondingly larger input and liquid is sufficient. sions suitable for process will entail output, but no alteration of principle.
Example 1 V An electrolyte, being'an aqueous solution containing per liter:
Grams M CI a 330 CaCl ca MgBr 12 NaCl 10 KCI '10 isfed to the cell through the pipe 40 at a rate (if-0.160
The current intensity is 055 'amp., the voltage 4 volt, the temperature is maintained at 20-30 CJby the circulation of water through the jacket 18. The anolyte is withdrawn'ata ratej of 0.126 liter per hour, and catholyteyat 'a rate off-0.034 liter 'p'er'ho'ur. During the electrolysis, magnesium-hydroxide precipitates, and owing to the-slope of thebottomf'Zl, the sediment coHects in the deepest 'zone of the cell'f'rom which it can be removed from time to-time by openingthe valve withdrawn has, purity 'of 99.5% and above. I V 7 The total quantity of 0.756 liter of anolyteiwithdrawn during 6 hours is shaken in a'separati'ng funnel'with '30 ml. of ethylene dibrornide, and the layers are "separated.
extractionis repeated twice each time with 30 ml. of and again twice'with 15 ml. each of ethylene dibromide.
About 8.3 grs. of bromine, viz.98%:of the"totaI bro-- mine produced, can be re'coveredfrom thecombined'extracts; V 7
At the end of the same period .the'total'weight of the magnesiumhydroxide precipitate is 2.76-grs.
The anodic current e'fiiciency is 95 the cathodic current efiiciency'85%.
Example2 The electrolyte is composed "as follows:
Grams/liter MgCl CaCl- 3s MgBr .j. 4 NaCl 80 Kcl 11 The other data are asin Example l ,'-but' the rate offeed of the electrolyte-is OISOOlit'erper'hOur, the 'anolyte withdrawal is 0.370 liter-perliour, the cathoylte withdrawal limited. .In all these exbromine concentration of a the aqueous liquorhas dropped to 4;8 grs.-'per'liter. The
0.130 liter per hour. Yield after 6 hours: 8.5 grs. of bromine, 2.76 grs. of Mg(OH) The anodic current efficiency is 95%, the cathodic current efiiciency 85%.
Example 3 The electrolyte is composed as follows:
Grams/liter NaCl 100 KBr l K CrO 2 The other data are as in Example 1, but the rate of feed of the electrolyte is 0.150 liter per hour, the anolyte withdrawal is 0.080 liter per hour, and the catholyte withdrawal 0.070 liter. With an anode current efiiciency of 45%, a total amount of 4.0 grs. of bromine is obtained from 0.480 liter of anolyte after 6 hours.
Example 4 The electrolyte is composed as follows:
Grams/liter NaCl 100 KBr 10 Example 5 The electrolyte is composed as follows:
Grams/liter NaCl 100 CaCl X131 The remaining data are as in Example 1, but the rate of feed of electrolyte is 0.150 liter per hour, the anolyte withdrawal is 0.108 liter per hour, and the catholyte withdrawal is 0.042 liter per hour.
After 6 hours, 5.4 grs. of bromine can be recovered from 0.648 liter of anolyte withdrawn. The anodic current cfliciency is 60%.
Example 6 The electrolyte is composed as follows:
Grams/liter NaCl 100 MgCl, 50 K1 1 The remaining data are as in Example 1, but the rate of feed of electrolyte is 2.442 liters per hour, the anolyte withdrawal is 1.642 liters per hour, and the catholyte withdrawal is 0.800 liter per hour.
At the end of 6 hours, 7.4 grs. of iodine can be recovered from 9.852 liters of anolyte withdrawn. The anodic current efliciency is 52%.
Example 7 End-brine from evaporation pans of ocean water was used as electrolyte. A partial analysis showed the following composition per liter:
Grams Ca++ 0.02 Mg 46.8 Cl- 200.7 Br- 2.1
The brine, like ocean water in general, has a pH above 7. An addition of acid is therefore required in order to prevent the formation of hypobromite. The acid may be added either to the incoming electrolyte or to the anolyte after electrolysis. If sulphuric acid is used, an addition of 0.85 gr. per liter of the above brine is sulficient. The other data are as in Example 1, but the rate of feed of the electrolyte is 0.80 liter per hour, the anolyte withdrawal is 0.750 liter per hour, the catholyte withdrawal 0.05 liter per hour. The total yield after 7 hours is 5.25 liters of anolyte containing 1.9 gr. of bromine per liter, and 0.35 liter of catholyte containing magnesium hydroxide. The quantity of bromine obtained corresponds to a current yield of We claim:
1. A continuous process for the electrolysis in a diaphragm-less cell of aqueous brines containing at least one salt selected from the group consisting of alkali and alkaline-earth bromides and iodides, comprising setting up in the electrolyte, by the passage of current, an electrochemically neutral zone as well as an anolyte zone and a catholyte zone, both of these latter zones being located above the neutral zone and merging into it, continually feeding fresh electrolyte brine into the neutral zone, thereby producing a continuous upward flow of electrolyte brine from the neutral zone into the anolyte zone and a. separate similar flow into the catholyte zone; and continuously withdrawing from the anolyte zone an aqueous liquor containing an elementary halogen of the group consisting of bromine and iodine, and spent liquor from the catholyte zone.
2. A process as claimed in claim 1, wherein the electrolyte brine contains magnesium chloride.
3. A process as claimed in claim 2, wherein the electrolyte contains alkali metal ions, magnesium ions in an amount at least equivalent to the alkali metal ions, chlorine and bromine ions, and the electrolysis is conducted at such a rate of flow of the electrolyte that the anolyte is an aqueous bromine solution and the major part of the alkali hydroxide formed at the cathode is converted into alkali halide with simultaneous formation of magnesium hydroxide.
4. A continuous process for the electrolysis in a diaphragm-less cell of aqueous brines containing at least one salt selected from the group consisting of alkali and alkaline earth bromides and iodides and in addition at least one chloride, comprising setting up in the electrolyte, by the passage of current, an electro-chemioally neutral zone as well as an anolyte zone and a catholyte zone, both of these latter zones being located above the neutral zone and merging into it; continually feeding fresh electrolyte brine into the neutral zone, thereby producing a. continuous upward flow of electrolyte brine from the neutral zone into the anolyte zone and a separate similar flow into the catholyte zone; and continuously withdrawing from the anolyte zone an aqueous liquor containing an elementary halogen of the group consisting of bromine and iodine, and spent liquor from the catholyte zone.
5. A process as claimed in claim 4, wherein the electrolyte contains chloride and bromide and the input rate of the brine is so regulated that the anolyte is an aqueous bromine solution.
6. A process as claimed in claim 4, wherein the electrolyte contains chloride and iodide and the input rate of the brine is so regulated that the anolyte is an aqueous iodine solution.
References Cited in the file of this patent Australia Nov. 4, 1938