FI59424B - FOERFARANDE FOER REDUCERING AV BILDNINGEN AV TJOCKT KVICKSILVER PAO BOTTENPLATTAN I EN KVICKSILVERCELL - Google Patents

Description

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England-England (GB) 23536/73 (71) Imperial Chemical Industries Limited, Imperial Chemical House, Mill Bank, London SW1, England-England (GB) (72) Denis Lee, Runcorn, Cheshire, Leslie Norburn, Runcorn, Cheshire,

England-England (GB) (7 ^) Oy Kolster Ab (5 * 0 Method for reducing the formation of a layer containing thick mercury in the base plate of a mercury cell - Förfarande för reducering av bildningen av tjockt kvicksilver pä bottenplattan i en kvicksilver-cell i

The invention relates to a method for reducing the formation of a layer comprising thick mercury in the bottom plate of a mercury cell comprising a flowing mercury amalgam cathode. i

Many countries around the world have extensive cellular equipment for producing chlorine and alkali hydroxide lye by electrolysis of an alkali metal chloride solution. In this case, the the solution is electrolyzed as it flows between the lower surfaces of the graphite or metal anode plates and the flowing liquid cathode by feeding mercury or dilute alkali metal amalgam to one end or the other side of the cell and removing the alkali metal enriched amalgam from the opposite end or opposite side of the cell. The chlorine generated by the anodes is then continuously removed from the top of the cell, and the liberated alkali metal that accumulates in the amalgam cathode stream is continuously transferred to 2,59424 amalgam concentrate and converted to an alkali hydroxide dilip by the reaction between the enriched amalgam and water in the recovery cell. Ko. the cell is commonly referred to as a denotation 1. rinsing cell, from which dilute amalgam is pumped to the electrolysis cell.

A very common problem with the use of such cells is the formation of mercury-containing deposits on the bottom plate of the cell. Such a layer is sometimes referred to as "mercury butter".

The formation of a mercury-containing layer is not fully known. It is thought to be caused by impurities in the brine electrolyte, but even if the brine is purified in the best possible way, a mercury precipitate can be formed when the cell is used for a longer period of time. This problem is now becoming quite topical, as high current densities have recently begun to be used.

Thick layers of mercury can cause power outages between the anodes and cathode amalgam. Such interruptions 1. short circuits can, in addition to reducing current efficiency, cause serious damage specifically to surface-treated metal anodes, which have recently begun to displace conventionally used graphite anodes.

We have now found that the formation of a layer of mercury on the bottom plate of the cell can be reduced by dispersing 1. by decomposing water or an aqueous substance in an amalgam cathode without interrupting the electrolysis.

The process according to the invention is characterized in that water or an aqueous medium is dispersed in the amalgam intermittently or continuously at one or more stations in the amalgam flow line without interrupting the electrolysis.

In this case, a saline solution, for example an aqueous electrolyte of the cell, is preferably used as the aqueous substance.

The accompanying drawings schematically illustrate seven suitable embodiments of a device with which the method according to the invention can be applied in practice.

In each of Figs. 1 to 6, an isometric projection of the inner part of a mercury-silver cathode electrolysis cell, seen from the other side edge of the base plate, is shown.

3 5 9 4 2 4 amalgam flow occurs in the longitudinal direction of the cell. Figure 7 is a diagrammatic longitudinal section of the mercury cathode electrolysis cell in the longitudinal direction. The middle part of the cell is then cut off. The figure also shows the amalgam recycling arranged outside the cell. Figure 8 shows a detail of Figure 7.

In each of Figs. 1 to 6, the bottom plate 1 of the cell has an Amalam current cathode 2 and an aqueous electrolyte 3 flowing above it. The anodes located in the electrolyte 3 above the amalgam layer are omitted from the figures for the sake of clarity.

Figure 1 shows an inclined flap 4 positioned transversely to the amalgam flow line. The lower edge of the flap is detached from the base plate 1 and is located below the normal level of the amalgam, while its upper part is in an aqueous electrolyte 3. The flap 4 may comprise, for example, a steel core surface-treated with a chlorine-resistant plastic material. Because po. the inclined flap causes interference in the flow of the above two liquids, and since, in addition, the specific gravity of the amalgam layer is very high, some of the aqueous electrolyte enters under the flap with the amalgam. Due to the vortex motion, enough electrolyte is decomposed into the amalgam, which in turn results in the mercury precipitate that tends to form on the bottom plate of the cell decompose at a considerable distance downstream of the flap. Due to the length of the cell and other factors, e.g. the slope of the base plate and the current density, several flaps 4 may have to be placed along the cell in order to keep it free of mercury deposits along its entire length.

In Fig. 2, the roll 5 replaces the flap 4 shown in Fig. 1.

The roller rotates on a bearing shaft 6 (bearings are not shown in the figure). The bearings can be attached to the bottom plate of the cell or to its walls. The roll is spaced from the base plate and immersed partly in the cathode amalgam 2 and partly in the aqueous electrolyte 3. The rotation of the rollers caused by the amalagam flow draws some of the aqueous electrolyte under the roll and causes sufficient degradation of the electrolyte in the amalgam. Additional rollers 5 can be placed at suitable distances in the amalgam flow line.

The electromagnetic transducer 7 shown in Fig. 3, which is preferably operated at a certain ultrasonic frequency, is placed in the amalgam flow line to replace the flap 4 shown in Fig. 1 and the roll 5 of Fig. 2. The lower surface of the transducer is adjusted to oscillate in its center position. by means of an aqueous electrolyte distribution surface when the transducer is not in operation. The transducer can be kept running continuously to maintain the dispersion of the aqueous electrolyte 1. decomposition in the amalgam downstream of the transducer, or the transducer can be operated intermittently only for short periods of time, e.g. 10 minutes, when the formation of mercury deposits can be mainly prevented. Additional transducers 7 can be installed at certain distances from each other in the amalgam flow line, if necessary, in order to keep the base plate free of mercury deposition along its entire length. In addition, each transducer shown in Figure 3 as a single transducer 7 can be replaced by a number of smaller, independently operating transducers placed side by side across the amalgam flow line. The transducer mounting devices and power lines shown at 7a in Fig. 3 may be suitably arranged to pass through the cell cover (not shown).

Figure 4 shows an alternative arrangement comprising electromagnetic transducers for dispersing an aqueous electrolyte in an amalgam cathode stream. The transducer 26 is then mounted on the cell cover (not shown) and the mechanical vibrations generated by the transducer are then transmitted to the cell by a rod 27 which passes through the cell cover and extends to the interface between the amalgam cathode 2 and the aqueous electrolyte 3. Although only one transducer is shown in Figure 4 for clarity, generally a plurality of transducers 26, each attached to a rod 27, are mounted a certain distance apart across the cell to disperse the aqueous electrolyte in the amalgam cathode over the entire width of the cell. If desired, a row of openable openings can be made in the cell cover, through which the rods 27 can be inserted into the cell only when the amalgam cathode is to be treated for only a short time. Once the treatment has taken place, the transducers can be transferred again to another model in the same cell, which also has a row of openings, or they can be transferred completely to another cell. The rods 27 must be of chlorine-resistant material or at least be surface-treated with a chlorine-resistant material. The most suitable method is to surface-treat the rods into an electrically insulating structure, so that no short circuit can occur between the anodes and the cathode of the operating cell when the rods are placed in the cell and lifted out of it.

Figure 5 shows an apparatus for supplying water or saline to an amalgam cathode by injecting the substance in question from a set of spray nozzles placed in a cell above the amalgam cathode 2. The inlet pipe 28 has a series of side pipes 29, and at each end there is a spray nozzle 30 (only one is shown in the figure). The tubes 28 can be attached to the cell above the aqueous electrolyte 3. It may also be outside the cell, with the side tubes 29 going into the cell through its cover (not shown in the figure). The spray nozzles 30 may be located above the aqueous electrolyte as shown in the drawing or may alternatively be arranged in the aqueous electrolyte. The end 31 of the tube 28 is closed and water or saline is fed in from the end 32 at a sufficient pressure so that the liquid jets out of the nozzles 30 at a rate sufficient to penetrate the aqueous electrolyte 3 and mix with the amalgam cathode 2. A suitable way is to arrange directing jets of water or saline through holes or openings in the anode plates or between adjacent anodes.

Figure 6 shows an arrangement for spraying water or saline directly onto the amalgam cathode. Attached to the cell, preferably above the plane of the flowing electrolyte 3, is an inlet tube 33 as shown in the figure, provided with a series of side tubes 34, each end having a spray nozzle 35 (only one shown in the figure) in the flowing Amalam cathode layer 2. The tube end 33 is closed, and water or saline is introduced from the end 37 at a sufficiently high pressure to be able to penetrate the amalgam cathode layer. Here, too, nozzles 35 are used.

In general, sufficient water decomposes into mercury, which, 59424, returns from the scrubber to the chlorine cell and prevents mercury from depositing for a short distance downward from the mercury feed port of the cell. Therefore, it is not necessary to disperse more water or saline into the amalgam cathode near the mercury feed port. Thus, the first position downstream of the inlet at which any of the devices shown in Figures 1 to 6 can be mounted is generally selected to be at the point e where the formation of mercury deposits is known to begin during normal use of the cell.

Figure 7 shows another device arrangement for decomposing water or saline into flowing cathode amalgam in a cell as required by the invention. The bottom plate 1 of the cell has a flowing amalgam cathode 2 and above it a similarly flowing brine electrolyte 3. The number 8 denotes the end walls of the cell through which the brine supply pipe 9, the spent brine outlet pipe 10, the mercury or dilute amalgam supply pipe 11 and the amalgam amalgam supply pipe 11 are guided. the cover 13 has a chlorine gas outlet 14. The electrical connections 15 to the anodes 16 pass through the cover. The enriched amalgam is continuously removed from the cell at point 12 to a rinsing device 17, where it is decomposed using water (not shown). A dilute amalgam stream flows from the rinsing device 17 ns. through the water wash tank 18 and the pump 19 back to the feed end of the cell along the pipe 20. The main stream containing the diluted amalgam returns to the cell through the valve 21 and the inlet 11. As already mentioned above, the mercury stream returning from the flushing device to the cell via the pipe 20 contains water decomposed therein. According to the embodiment shown in Figure 7, only a fraction of this "wet" mercury stream, controlled by valves 22 and 21, is fed along line 23 to applicator rod 24. It is located transverse to the α-amalgam flow line and protrudes into the amalgam cathode just upstream of the known mercury-containing deposits. cell in normal use. Figure 8 shows the spreading bar in a side view. The stream of "wet" mercury coming through the tube 23 is directed to the hollow applicator rod 24 and passes to the amalgam cathode flowing through the nozzle set 25. Additional applicator rods and 24 connected to the side tubes from the tube 23 (not shown in Fig. 59424 7) may be installed in the cell at predetermined distances from the amalgam stream to keep the base plate free of mercury deposition along its entire length.

As a variant of the structure according to the invention described in the previous paragraph, more water or brine (preferably non-chlorinated brine) can be decomposed into mercury which returns from the rinsing device to the cell; for example, by installing an emulsifier to which the selected aqueous substance is fed either in the total mercury stream flowing in tube 20 shown in Figure 7 (Figure 7, item A) or in a smaller 1. side stream flowing in tube 23 (Figure 7, item B). This variant has the advantage of compensating for the progressive separation of the dispersed water from the mercury stream in the piping, which is otherwise due to the large difference in specific weights. In addition, the the structural variation results in an aqueous phase 1 phase with a higher degree of concentration at the application points. Once a plurality of applicator rods 24 have been installed in the cell, a separate emulsifier B can be installed in all side tubes of the tube 23 which "feed" the separate applicator rods, if desired.

The term "emulsifier" as used herein means a mechanical device having a unit with rotating vanes in a fixed, torn plate and positioned across a interface between the amalgam and the water to be added to absorb water into the amalgam by means of rotating vanes.

The invention is illustrated but not limited by the following examples.

Example 1

The rate of mercury layer formation in the mercury cell was determined as follows: The vertically adjustable probe 1. probe (diameter 3 mm) was inserted into the cell through an opening in its cover and screwed toward the base plate. Using a circuit comprising a voltmeter, the probe was then connected to the base plate of the cell. The first reading from the micrometer screw was recorded when the probe touched the top surface of the mercury as shown by the voltmeter. Another reading of the micrometer screw was recorded again after the probe came into mechanical contact with the cell base plate. Em. the difference between the readings (about 5 ... 8 mm) indicated the thickness of the mercury film. The measurement could be performed with an accuracy of _ + 0.05 mm.

8 59424

Several measurements, e.g., 20 ... 30, were performed at different points in the cell, and the average thickness of the mercury film was calculated at each point. The measurements were repeated at regular intervals for five periods of 5 to 6 days, and the rate of increase in the thickness of the mercury film and the rate of formation of the mercury layer were made.

A mercury cell that operated at 180,000 amp. stream, fed with 25.5 wt% sodium chloride brine at 4 m / h. The average mercury flow rate was 53 1 / min. Water was sprayed into the pump 19 (shown in Fig. 7) at a rate of approximately 60-95 l / h.

For comparison, the mercury cell was used with respect to the brine flow and the electric current under the same conditions mentioned above, so that the average mercury flow rate was 47 l / min. without spraying any water. Both measurements were performed over seven 5 to 8 day periods.

The average velocities of mercury precipitate formation at different points in the cell with and without water jet are shown below:

During the formation of the mercury

Distance shoe speed (mm / day) in lift (cm) water spray without water spray 380 0.018 0.035 760 0.034 0.355 1140 0.382 0.450

The water content and water particle size of the amalgam were measured by allowing the water particles to rise to the surface of the amalgam and then measuring the amount of water that accumulates on the surface at various time intervals. When water spraying was not used, the mercury feed into the cell contained 0.02 to 0.1 ppm by weight of water with a particle size of less than 12 microns (Stokes diameter). When water spraying was used, the mercury feed contained 0.2 to 0.6 ppm by weight of water with a particle size of less than 12 microns (Stokes diameter). These measurements confirmed that spraying water into the pump had increased the water dispersion in the amalgam feed of the cell (there was already some water in this amalgam).

Example 2

An emulsifier (point A in Figure 7) was connected to the cell-fed mercury feed. The mercury cell was used for brine flow and electric current under the same conditions as in Example 1. The average mercury flow rate was 9,594,241, 62 Ι / min, and the experiment was performed over five 5-9 day periods, but using an emulsifier instead of spraying water into the pump. The type of emulsifier described above is manufactured by Silverson Machines Limited, England.

For comparison, the mercury cell was used for brine flow and electric current under the same conditions as above, with an average mercury flow rate of 10 Ι / min, and the experiment was performed for five periods of 5 to 9 days without an emulsifier.

The average rates of mercury precipitation observed at different points in the cell with and without the emulsifier are shown below:

Average distance of mercury precipitate formation (Iran / day) in lift (cm) emulsifier without emulsifier 190 0 0.015 380 0.009 0.065 570 0.011 0.072 760 0.085 0.193 945 0.145 0.284

The measurements described in Example 1 showed that the e-mulching device had increased the water dispersion in the mercury feed of the cell. Thus, when the emulsifier was not used, the cell mercury feed contained 0.1 to 0.4 ppm by weight of water with a particle size of less than 12 microns (Stokes diameter). When using an emulsifier, the mercury feed again contained 1 to 4 ppm by weight of water with a particle size of less than 12 microns (Stokes diameter).

Claims (6)

10 59424 A method for reducing the formation of a layer containing thick mercury in a base plate (1) of a mercury cell comprising a flowing mercury amalgam cathode (2), characterized in that water or an aqueous medium is dispersed in the amalgam intermittently or continuously at one or more stations. Process according to Claim 1, characterized in that the aqueous substance is brine. Method according to Claim 1 or 2, characterized in that the water or the aqueous solution is dispersed in part or all of the amalgam stream which returns from the rinsing device (17) to the cell. Process according to Claim 3, characterized in that the water or the aqueous solution is dispersed in part or in the entire amalgam stream using an emulsifier (A) located between the rinsing device (17) and the cell. A method according to claim 3, characterized in that the water or the aqueous solution is dispersed in the entire amalgam stream by spraying water into an amalgam pump (19) located between the rinsing device (17) and the cell. Method according to one of Claims 3 to 5, characterized in that a part of the amalgam containing water or an aqueous solution is introduced into the feed end (11) of the mercury cell and a part is introduced into the amalgam stream inside the cell via a spreading rod (24).