US3905880A

US3905880A - Operation of mercury-cathode cells

Info

Publication number: US3905880A
Application number: US465416A
Authority: US
Inventors: Denis Lee; Leslie Norburn
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1973-05-17
Filing date: 1974-04-29
Publication date: 1975-09-16
Anticipated expiration: 1992-09-16
Also published as: CA1067449A; GB1437472A; FI59424B; ZA742933B; FI59424C; IT1012423B; JPS5049197A; FI152974A; DE2423996A1; NL7406441A; BE815178A; FR2229461A1; ES426408A1; AU6854774A; FR2229461B1

Abstract

A process for the reduction of thick mercury on the baseplate of a mercury cell which comprises dispersing water or an aqueous medium (preferably brine) continuously or intermittently at one or more stations along the line of amalgam flow without interruption of the electrolysis. It is preferred to inject water or brine in part or whole of the stream of amalgam returning from the denuder to the cell, for example by means of an emulsifier located between the denuder and the cell or by injecting water into the amalgam pump.

Description

United States Patent 1191 Appl. No.: 465,416

Foreign Application Priority Data May 17, 1973 United Kingdom 23356/73 References Cited UNlTED STATES PATENTS 7/1900 Entz 204/99 4/1934 Ramsey et a1. 204/250 Lee et al. Sept. 16, 1975 [54] OPERATION OF lVlERCURY-CATHODE 2,307,835 1/1943 Gardiner 204/99 CELLS 3,560,355 2/1971 Shibata et a1. 204/99 3,627,652 12/1971 De Nora 204/99 1 Inventors: Denis Leslie Norbum, both of 3,645,866 2/1972 Volkov 204 99 Runcorn, England [73] Assignee: Imperial Chemical Industries Primary Examiner-R Andrews Limited, London, England Attorney, Agent, or FirmCushman, Darby &

Cushman [22] Filed: Apr. 29, 1974 [5 7] ABSTRACT A process for the reduction of thick mercury on the baseplate of a mercury cell which comprises dispersing water or an aqueous medium (preferably brine) continuously or intermittently at one or more stations along the line of amalgam flow without interruption of the electrolysis.

It is preferred to inject water or brine in part or whole of the stream of amalgam returning from the denuder to the cell, for example by means of an emulsifier located between the denuder and the cell or by injecting water into the amalgam pump.

11 Claims, 8 Drawing Figures PATENTED SEP I 6 I975 SHEET 2 BF 4 PATENTEH SEP 3 8 E92 5 snmaurg The present invention relates to the operation of mercury-cathode cells for the electrolysis of alkalimetal chloride solutions. More particularly it relates to stallations of cells for the manufacture of chlorine and caustic alkali by the electrolysis of alkali-metal chloride solution, wherein the said solution is electrolysed while flowing between the lower faces of an array of graphite or metal anode plates and a flowing liquid cathode, which is maintained by feeding in mercury or dilute alkali-metal amalgam at one end or one side of the cell and withdrawing amalgam enriched in alkali-metal at the opposite end or side of the cell. Chlorine liberated at the anodes is continuously removed from the top of the cell and the liberated alkali-metal, which collects in the flowing amalgam cathode is continuously removed in the enriched amalgam and converted to caustic al' kali by reaction of the enriched amalgam with water in a soda cell, usually called a denuder, from which dilute amalgam is recirculated by means of a pump to the electrolytic cell.

A well-known problem in operating such cells is the build-up of deposits of thick mercury, sometimes referred to as mercury butter on the baseplate of the cell. The mechanism of thick mercury formation is not fully understood. It is thought to be influenced by the presence of trace impurities in the brine electrolyte, but even with the best practicable attention to purification of the feed-brine thick mercury deposits can build up with prolonged operation of the cell and the problem has tended to become now acute with the high-current-density operation which has been practised in recent times.

Thick mercury deposits can cause current shorts between the anodes and the cathode amalgam. Besides reducing the current efficiency of the process such shorts can cause serious damage, especially to the coated metal anodes which have recently begun to replace conventional graphite anodes.

We have now found that the build-up of thick mercury on the cell baseplate can be reduced by dispersing water or an aqueous medium in the amalgam cathode without interruption of the electrolysis.

According to the present invention we provide a process for the reduction of thick mercury on the cell baseplate of a mercury cell which comprises dispersing water or an aqueous medium continuously or intermittently at one or more stations along the line of the amalgam flow without interruption of the electrolysis.

The preferred aqueous medium is brine, for example the aqueous electrolyte of the cell.

The accompanying drawings illustrate schematically seven suitable embodiments of apparatus for putting the process of the invention into practice.

Each of the FIGS. 1 to 6 shows an isometric projection as seen from one side edge of the baseplate the interior of a portion of a mercury-cathode cell in which the cathode-amalgam flow is along the length of the cell. FIG. 7 shows a schematic vertical section along the length of a mercury-cathode cell with its midportion cut out and the external amalgam circulation. FIG. 8 shows a detail of- FIG. 7.

In each of the FIGS. 1 to 6 the cell baseplate l carries the flowing amalgam cathode 2 and above this is the flowing aqueous electrolyte 3. The anodes which lie above the amalgam layer in the electrolyte 3 have been omitted from the drawings for the sake of clarity.

In FIG. 1 an inclined baffle 4 is fixed across the line of amalgam flow with its lower edge spaced from the baseplate l and below the normal level of the amalgam, and with its upper portion lying in the aqueous electrolyte 3. The baffle 4 may suitably consist of a steel core coated with a chlorineresistant plastics material. Because of the interference to flow of the two liquids by the inclined baffle and the very high specific gravity of the amalgam layer, a portion of the aqueous electrolyte is drawn under the baffle with the amalgam, turbulence is created and sufficient electrolyte is caused to disperse in the amalgam to cause decomposition of any thick mercury tending to deposit on the baseplate for an appreciable distance downstream from the baffle. Depending on the length of the cell and other parameters, such as the slope of the baseplate and current density, a plurality of baffles 4 spaced apart along the cell may be required to keep the whole length of the cell free from thick mercury deposits.

In FIG. 2 a roller 5 replaces the baffle 4 of FIG. 1. The roller is rotatable about an axle 6 carried in bearings (not shown), which may be fixed to the baseplate or the walls of the cell. The roller is spaced away from the baseplate and is immersed partly in the cathode amalgam 2 and partly in the aqueous electrolyte 3. R0- tation of the rollers caused by'the flowing amalgam layer draws a portion of the aqueous electrolyte beneath the roller and produces the required dispersion of electrolyte in the amalgam. Additional rollers 5 may be fixed at intervals down the line of the amalgam flow.

In FIG. 3 an electromagnetic transducer 7, working preferably at an ultrasonic frequency, is placed across the line of amalgam flow to replace the baffle 4 of FIG. 1 and the roller 5 of FIG. 2. The position of the'lower surface of the transducer is adjusted so as to oscillate about a mean position defined by the interface between the amalgam layer and the aqueous electrolyte when the transducer is not in operation. The transducer may be operated continuously to maintain a dispersion of aqueous electrolyte in the amalgam downstream from the position of the transducer or it may be operated for 7 short periods, eg 10-minute periods, intermittently so as to prevent the build-up of thick mercury deposits to any significant extent. Additional transducers 7 may be installed at intervals down the line of the amalgam flow as necessary to maintain any length of baseplate free from thick mercury deposits. Furthermore, each transducer shown in FIG. 3 as a single transducer 7 may be replaced by a plurality of smaller independently actuated transducers operating side by side across the line of amalgam flow. The suspension arrangements and power supply lines for the transducers, indicated as 7a, may suitably pass through the cover of the cell (not shown).

In FIG. 4 is shown an alternative arrangement employing electromagnetic transducers for dispersing aqueous electrolyte in the flowing amalgam cathode. Here a transducer 26 is installed above the cover of the cell (not shown) and the mechanical vibrations generated by the transducer are conveyed into the cell by means of a bar 27 passing through the cell cover and reaching to the interface between the amalgam cathode 2 and the aqueous electrolyte 3. Although for clarity only one transducer is shown in FIG. 4, generally a plu rality of transducers 26, each with its attached bar 27, v

will be installed in a spaced row across the cell so as to disperse aqueous electrolyte in the amalgam cathode over the full width of the cell. If desired, the cell cover may be provided with a row of openable ports through which the bars 27 may be introduced only when it is desired to treat the amalgam cathode for a short period. After the treatment the transducers may be moved to another station in the same cell, where another row of openable ports is provided, or to a different cell. The bars 27 must be made of chlorine-resistant material or must be coated with a chlorine-resistant material. Most suitably the bars are provided with an electricallyinsulating coating so as to avoid the possibility of shorting between the anodes and the cathode of the working cell when the bars are being introduced and removed.

FIG. 5 shows an arrangement for introducing water or brine into the amalgam cathode by injection from a series ofjet orifices placed above the amalgam cathode 2 in the cell. An inlet pipe 28 carries a series of branches 29, each of which is terminated by a jet orifice 30 (one only shown). The pipe 28 may be fixed within the cell above the aqueous electrolyte 3; alternatively it may lie outside the cell and the branches 29 will then pass into the cell through the cell cover (not shown). The jet orifices 30 may be placed above the aqueous electrolyte as shown in the drawing or alternatively they may be placed within the aqueous electrolyte. The end 31 of pipe 28 is closed and water or brine is fed in at the end 32 under adequate pressure so that it issues from the jet orifices 30 with sufficient velocity to penetrate through the aqueous electrolyte 3 and disturb the amalgam cathode 2. It may suitably be arranged for the high-speed jets of water or brine thus produced to pass through holes or slots provided in the anode plates or through gaps provided between neighbouring anodes.

FIG. 6 shows an arrangement for injecting water or brine directly into the amalgam cathode. An inlet pipe 33 is fixed in the cell, preferably above the level of the flowing electrolyte 3 as shown, and carries a series of branches 34, each of which terminates in a jet orifice 35 (only one shown) within the flowing amalgam cathode layer 2. The end 36 of pipe 33 is closed and water or brine is fed in at the end 37 under sufficient pressure to emerge into the amalgam cathode layer through the jet orifices 35.

In general sufficient water remains dispersed in the mercury returning to the chlorine cell from the denuder to prevent the build-up of thick mercury deposits for a short distance downstream from the mercury inlet of the cell. It is not therefore necessary to disperse additional water or brine in the amalgam cathode near the mercury inlet and the first station downstream from the inlet at which any of the devices shown in FIGS. 1 to 6 is installed will generally be chosen to lie just before the position at which the build-up of thick mercury deposits is known to start during conventional operation of the cell.

In FIG. 7 is shown another arrangement of apparatus for dispersing water or brine in the flowing cathode amalgam within the cell in accordance with the invention. The cell baseplate 1 carries the flowing amalgam cathode 2 and above this is the flowing brine electrolyte 3. 8 are the end walls ofthe cell through which pass the brine inlet 9, the spent-brine outlet 10, an inlet 11 for mercury or dilute amalgam and an outlet 12 for enriched amalgam. The cell cover 13 is provided with an outlet 14 for chlorine gas, and the electrical connections 15 pass through the cover to the anodes 16. Enriched amalgam is continuously removed from the cell at 12 to the denuder 17, where it is decomposed by a supply of water (not shown), and a stream of dilute amalgam flows from denuder 17 through water-wash box 18 and pump 19 back to the inlet end of the cell through pipe 20. The main stream of dilute amalgam returns to the cell through valve 21 and inlet 11. As discussed hereinbefore, the mercury stream returning to the cell from the denuder by way of pipe 20 contains some water dispersed in it. According to the embodiment of the invention illustrated in FIG. 7, a small fraction of this wet mercury stream, controlled by valve 22 in conjunction with valve 21, is fed by way of pipe 23 to a spreader-bar 24 which lies across the line of amalgam flow and dips into the amalgam cathode just upstream from the position where build-up of thick mercury deposits is known to start during conventional operation of the cell. The detail of the spreader-bar is shown in side elevation in FIG. 8. The stream of wet mercury delivered through pipe 23 passes into the hollow spreader-bar 24 and enters the flowing amalgam cathode through a series of orifices 25. Additional spreader-bars 24 connected to branches (not shown) from pipe 23 may be installed in the cell at intervals further down the line of amalgam flow to maintain any length of baseplate free from thick mercury deposits.

As a modification of the embodiment of the invention discussed in the preceding paragraph, additional water or brine (preferably unchlorinated feed-brine) may be dispersed in the mercury returning to the cell from the denuder, for instance by installing an emulsifier fed with the chosen aqueous medium either in the total mercury stream flowing in line 20 of FIG. 4 as indicated at A or in the minor stream flowing in line 23 as indicated at B. This modification has the advantage of compensating for the progressive separation of dispersed water from a mercury stream as it moves along the pipelines, caused by the great difference in specific gravities, and ensures a higher concentration of aqueous phase at the points of application. If desired, when a plurality of spreader-bars 24 is installed in the cell, a separate emulsifier B may be installed in each of the branches of line 23 which feed the individual spreaderbars.

By the term emulsifier, we mean a mechanical device consisting of rotating vanes fitting within a stationary perforated screen, and which is positioned across the interface between the amalgam and the water to be added so as to draw water into the amalgam when the vanes are rotated.

The invention is further illustrated but not limited by the following Examples.

EXAMPLE I The rate of formation of thick mercury content in a mercury cell was determined as follows. A vertically adjustable copper probe (diameter 3 mm) was inserted through a hole in the cell cover and screwed down towards the baseplate. The probe was connected by means of an electric circuit including a voltmeter to the cell baseplate. A first reading on a micrometer gauge was taken when the probe made contact with the upper 8 mm) indicated the thickness of the mercury film and could be measured to an accuracy of 1.005 mm. Sevnute, over five periods of 5 to '9 days each, but without the emulsifier.

The observed mean rates of formation of thick men cury at different positions along the cell, with and with 5 out the emulsifier. are shown below:

eral measurements eg 20 to 30, were taken at different positions along the cell and the mean thickness of the mercury film in each position was calculated. The measurements were repeated at regular intervals'over 5 periods of 5 to 6 days each and the rate of increase of the thickness of the mercury film was equated with the rate of formation of thick mercury.

A mercury cell operating with a current of 180,000 amp was fed with 4 m lhour of 25.5 percent 7w sodium chloride brine. The mean mercury flow rate was 53 litres/minute. Water was injected into the pump 19 (shown in FIG. 7) at a rate in the range 60 to 95 litres/- hour.

For the purposes of comparison, the mercury cell was operated under the same conditions of brine flow and current, with a mean mercury flow rate of 47 litres/minute, but without water injection. The measurements were carried out seven periods of 5 to 8 days each.

The observed mean rates of formation of thick mercury at different positions along the cell, with and without water injection, are shown below:

Distance along Mean rate of formation of The water content of the amalgam and the particle size of the water was measured by allowing water particles to rise to the surface of the amalgam and measuring the amount of water collecting at the surface at various time intervals. When there was no water injection, the mercury feed to the cell contained 0.02 to 0.1 ppm by weight of water whose particle size was less than 12 micron Stokes diameter; when water was injected, the mercury feed contained 0.2 to 0.6 ppm by weight of water whose particle size was less than 12 micron Stokes diameter. These measurements confirmed that injection of water into the pump had produced an increased amount of dispersion of water in the amalgam feed to the cell (there was already some water present in this amalgam).

EXAMPLE 2 An emulsifier (as shown at A in FIG. 7) was incorporated in the mercury feed to the cell. The mercury cell was operated under the same conditions of brine flow and current as in Example I. with a mean mercury flow rate of 62 litres/minute, over five periods of 5 to 9 days each, but using the emulsifier instead of water injection into the pump. The emulsifier, which was of the type described hereinbefore was manufactured by Silverson Machines Limited, England.

For purposes of comparison, the mercury cell was operated under the same conditions of brine flow and current, with a mean mercury flow rate of 10 litres/mi- Distance along Mean rate of formation of the cell (feet) thick mercury (mm/day) with emulsifier without emulsifier 3l O.l45 0.284

Measurements as described in Example 1 showed that the emulsifier produced an increased amount of dispersion of water in the mercury feed to the cell. Thus, when the emulsifier was not in use, the mercury feed to the cell contained 0.1 to 0.4 ppm by weight of water whose particle size was less than 12 micron Stokes diameter; when the emulsifier was used, the mercury feed contained 1 to 4 ppm by weight of water whose particle size was less than 12 micron Stokes diameter.

What we claim is:

1. A process for the reduction of thick mercury on the cell baseplate of a mercury cell which comprises dispersing water or an aqueous medium within the body of the amalgam intermittently or continuously at one or more stations along the line of the amalgam flow without interruption of the electrolysis.

2. A process as claimed in claim 1 wherein the aqueous medium is brine.

3. A process as claimed in claim 1 wherein water or the aqueous solution is dispersed in part or whole of the stream of amalgam returning from a denuder to the cell.

4. A process as claimed in claim 3 wherein water or the aqueous solution is dispersed in part or whole of the stream of amalgam by means of an emulsifier located between the denuder and the cell.

5. A process as claimed in claim 3 wherein the water or the aqueous solution is dispersed in the whole of the stream of amalgam by injecting water into a pump for the amalgam located between the denuder and the cell.

6. A process'as claimed in claim 3 wherein at least a portion of the stream of amalgam containing dispersed water is introduced along the line of amalgam flow within the cell.

7. A process as claimed in claim 2 wherein the dispersion of brine in the amalgam is effected in the cell by means of an inclined baffle fixed across the line of amalgam flow with its lower edge spaced from the baseplate and positioned below the normal level of the amalgam cathode and with its upper portion lying in the aqueous electrolyte.

8. A process as claimed in claim 2 wherein the dispersion of brine in the amalgam is effected in the cell by means ofa roller which is rotatable about an axis transverse to the line of amalgam cathode flow and which is spaced away from the baseplate so as to be immersed partly in the amalgam cathode and partly in the aqueous electrolyte.

9. A process as claimed in claim 2 wherein the disper sion of brine in the amalgam cathode is effected in the cell by means of a source of mechanical vibration.

10. A process as claimed in claim 1 wherein water or 11. A mercury cell having means for reducing the brine is dispersed in the amalgam by injecting a stream build-up of thick mercury on the cell baseplate which of brine or water into or above the aqueous electrolyte operates using the process of claim 1. or into the amalgam cathode itself.

I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3.905.880 Dat d Sept. 16. 1975 Patent No.

Inventor(s) Denis Lee, and Leslie Norburn It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE HEA DING At paragraph [30] concerning Foreign Application Priority Data please change the UK Application Number 3356/73" to read --23536/73--- Signed and Scaled this seventeenth Day Of February 1976 [SEAL] Attest:

C. MARSHALL DANN Commissioner ufPaIents and Trademarks RUTH C. MASON Arresting Officer

Claims

2. A process as claimed in claim 1 wherein the aqueous medium is brine.

6. A process as claimed in claim 3 wherein at least a portion of the stream of amalgam containing dispersed water is introduced along the line of amalgam flow within the cell.

8. A process as claimed in claim 2 wherein the dispersion of brine in the amalgam is effected in the cell by means of a roller which is rotatable about an axis transverse to the line of amalgam cathode flow and which is spaced away from the baseplate so as to be immersed partly in the amalgam cathode and partly in the aqueous electrolyte.

9. A process as claimed in claim 2 wherein the dispersion of brine in the amalgam cathode is effected in the cell by means of a source of mechanical vibration.

10. A process as claimed in claim 1 wherein water or brine is dispersed in the amalgam by injecting a stream of brine or water into or above the aqueous electrolyte or into the amalgam cathode itself.

11. A mercury cell having means for reducing the build-up of thick mercury on the cell baseplate which operates using the process of claim 1.