FR2476147A1 - METHOD FOR ELECTROLYSIS OF AQUEOUS ALKALI METAL HALIDE SOLUTION IN A MERCURY CATHODE CELL - Google Patents

Abstract

1. Process for the electrolysis of an aqueous alkali metal halide solution in an electrolysis cell of which the cathode is a sheet of mercury (11) flowing on an inclined baseplate (5) of the cell, in which the baseplate is caused to vibrate, characterized in that distinct zones (27, 28, 29, 30, ... 31, 32, 33) of the baseplate (5) are caused to vibrate separately and successively from the downstream end to the upstream end of the sheet of mercury (11), and the vibrations applied to each of the said distinct zones are controlled.

Description

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  The present invention relates to a method of electrolysis of an aqueous solution of alkali metal halide in a mercury cell.

  an aqueous solution of alkali metal halide, in particular

  to bind a brine of sodium chloride in an electro-

  lyse whose cathode is formed of a sheet of mercury

  on an inclined floor. An important difficulty encountered in the operation of mercury cathode electrolysis cells lies in the inadvertent formation of viscous clusters adhering to the sole of the cells and usually referred to as "heavy mercury" or "butter.

mercury".

  The formation of these clusters of large mercury adhering to the sole

  Electrolytic cells are poorly known and are particularly

  badly to the good functioning of these. These clusters of large mercury disturb the regular flow of the mercury cathode on the floor with, as a detrimental consequence, the possibility of fortuitous short circuits between mercury and

  anodes and a release of hydrogen into the cell.

  To remedy these drawbacks, it is proposed in Belgian Patent No. 800,615 of June 7, 1973 in the name of the Applicant, to periodically vibrate the sole of the cell, so as to

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  detach the large mercury clusters adhering to the sole and facilitate

  their evacuation out of the cell by training with the

current of mercury or amalgam.

  It has now been found that the efficiency of this known process can be greatly improved by vibrating the sole of the -

  cell according to a particular method.

  The invention therefore relates to a method for the electrolysis of an aqueous solution of alkali metal halide in a cell

  electrolysis, the cathode of which is a sheet of mercury

  on an inclined floor of the cell, in which the floor is vibrated; according to the invention, vibrating separately

distinct areas of the sole.

  In carrying out the process according to the invention, the vibrations used must be both sufficient to detach the large mercury clusters which adhere to the sole of the cell in the zone in question, but insufficient to cause droplet projections. of mercury in the cell or inadvertent contacts of the mercury layer with the anodes of

the cell.

  The choice of the frequency and the amplitude of the vibrations therefore depends on a large number of factors, among which include in particular the nature of the material of the sole and its roughness, the viscosity of the mercury layer, as well as its thickness. and

  its speed of flow on the sole, the direction of the vibrations.

  It must be established in each particular case by routine work. Vibrations can be parallel to the floor or present

  a component perpendicular to it. In the case of vibra-

  with a component parallel to the sole, this component

  Vibratory health can be parallel, perpendicular or oblique

  relative to the flow direction of the mercury web.

  However, according to the invention, it is preferred to use vibrators

  tions having a vertical component or perpendicular to the hearth.

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  In general, the method used to vibrate the sole of the cell is not critical. For example, it is possible to use either vibrators with mechanical or electromagnetic operation, such as those commonly used in screening techniques for ores and coals. The vibrators can be embedded in the mercury layer and applied against the upper face of the sole. For reasons of convenience, however, it is preferred to have the vibrators

  against the underside of the sole.

  In general, in the case of industrial electrolysis cells, good results are usually obtained with vertical vibrations developing forces on the hearth, of the order of 10 000 to 150 000 N, preferably 20 000 at 75 000 N, at a frequency between 50 and 1000 Hz, preferably between

150 and 300 Hz.

  The term "vibration zone" refers to an area of the surface of the hearth, which is subjected, independently of the remaining surface of the hearth, to vibrations of sufficient intensity and frequency to detach the clusters of fat mercury that could adhere anywhere in the area and allow them to

  entrainment by the mercury layer, outside said zone.

  In the method according to the invention, the number of vibration zones should preferably be sufficient so that together they cover the entire sole of the cell. In

  Alternatively, several areas may overlap.

  The optimum choice of the number and dimensions of the vibration zones depends on a variety of factors, including the length of the cell, the thickness of the mercury layer and its viscosity and rate of flow on the hearth, nature of the hearth material as well as its roughness and elasticity, the slope of the sole, the importance of the formation

  mercury and the choice of frequency and inten-

  vibration. It can be easily established in every

  particular case by routine work.

  In general, in the case where the vibration zones are arranged one behind the other between the upstream and downstream ends of the mercury layer, good results are usually obtained when the length of each zone is approximately between 0.5 and 10 m, preferably between 2 and 7 m. According to a first embodiment of the invention, several distinct zones of the cell sole can be vibrated simultaneously by using vibration characteristics which vary from one zone to another, for example frequencies or

  different vibration amplitudes. -

  However, according to another embodiment of the invention, it is preferred to vibrate successively several distinct zones of the hearth, which are arranged one behind the other, between the upstream end and the downstream end. of the sheet of mercury. This embodiment of the invention generally has the advantage of allowing a regular evacuation of the large mercury towards the downstream end of the cell, without disturbing the normal flow of the mercury layer nor the operation

normal of the cell.

  In the embodiment of this preferred embodiment of the invention, the optimum time interval that must be respected between the vibration of an area and the vibration of the next zone is that set by the large mercury clusters. for

  move from the area just vibrated to the next area.

  In general, a time interval approximately equal to the time taken by the mercury layer is advantageously chosen to cover the length of the zone that has just been

vibrate.

  A first embodiment of this preferred embodiment of the invention consists, for example, in vibrating the zones of the hearth, successively from the upstream end.

  to the downstream end of the mercury web.

  In practice, however, it has been observed that it is preferable to vibrate the hearth zones successively from the downstream end to the upstream end of the mercury web. In one embodiment of the process according to the invention, which has proved to be particularly advantageous, the hearth zones are vibrated successively from the downstream end to the end.

  upstream of the mercury layer, and then make them vibrate

  sively from the upstream end to the downstream end.

  All other things being equal, this mode of execution

  method of the invention makes it possible to optimize the evaluation

  cuation of the large mercury out of the cell, bringing to a

  minimum value the risk of a disturbance of the electro-

  lysis. Features and details of the invention will emerge from

  the following description of a particular embodiment

  of the invention, with reference to the accompanying drawing.

  Figure 1 is a schematic longitudinal elevational view, partially broken away, of a mercury cell electrolysis cell, designed for the application of the method according to the invention; FIG. 2 is a schematic representation in plan of the distribution of the zones of vibrations of the sole of the cell of

Figure 1.

  In these figures, the same reference numerals designate

identical elements.

  The electrolysis cell shown in FIG. 1 comprises a trough 1 closed by a cover 2. The trough 1 is delimited by longitudinal struts 3, transverse end walls 4 and a steel sole 5 having a moderate slope. ,

  for example of the order of 6 mm per running meter.

  The trough 1 rests, via the sole 5, on resilient supports 18 fixed to a base 19 generally made of concrete. Anodes 8 are suspended in the cell, vis-à-vis the sole 5, by rods 9 sealingly passing through the lid 2 and also playing the role of electrical conductors

at the anodes.

  The cell is connected, near the upstream end of its hearth 5, to a mercury inlet duct 10 intended to form the cathode 11 flowing on the hearth 5. Near its end

  downstream, the cell is in communication with an evacuation

  12 of the alkali metal amalgam formed by electrolysis of an aqueous solution of this alkali metal chloride between the anodes 8 and the cathode 11. This solution, for example a brine of sodium or potassium chloride, is introduced into the cell via an intake duct 13, while the solution exhausted after the electrolysis, is withdrawn from the cell through a discharge duct 14. The cell is also provided with a conduit 15 for the evacuation of chlorine cleared at the anodes. A series of vibrators 20, 21, -22, 23, ..., 24, 25, 26 are aligned between the upstream end and the downstream end of the hearth 5 and applied against the underside of the -this. Their respective positions and their orientation are chosen according to their power, so that when they are actuated simultaneously, they are capable of subjecting the entire sole 5 to vertical vibrations of sufficient intensity to detach masses of large mercury. normally adhering to the sole, but insufficient to cause droplets of mercury droplets in the cell or inadvertent contact with mercury

with the anodes.

  Taken in isolation, each vibrator 20, 21, 22, 23, ..., 24, 26 controls in this way a limited area of the hearth 5 of the

  cell; these areas, which are contiguous, are schematized

  27, 28, 29, 30, ..., 31, 32, 33 in FIG. 2. By definition, each of these zones, for example zone 29, is the part of the surface of the hearth 5 from which wholesale clusters

  mercury can normally be detached and evacuated by

  in the mercury layer 11, by actuation only of the vibrator disposed in line with said zone, in this case the

  vibrator 22 in the case of zone 29.

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  In practice, a satisfactory result can usually be achieved.

  making maintaining a uniform gap of about 3 to 5 m between two successive vibrators, such as 21 and 22 for example, so that the length of the vibration zones are also of this order of magnitude. The vibrators 20, 21, 22, 23,..., 24, 25, 26 may advantageously be mechanically operating vibrators capable of subjecting the sole 5 of the cell to vertical forces from below upwards, for example percussion. , of the order

  from 40,000 to 60,000 N, at a frequency of the order of 140 to 250 Hz.

  Alternatively, one can also make use of vibrators with higher frequency, for example of the order of 300 Hz, developing lower forces, for example of the order of 25 000 N. Mechanical vibrators usable in the the scope of the invention are those of the eccentric or unbalanced type,

  generally equipping vibrating screens (Techniques de l'Inge-

  General, A 904-5, July 1964, para.4, 72). Another category of mechanically functioning vibrators which are well suited to the invention is formed of those which are usually fitted to impact screens, and in which percussion masses are projected from bottom to top against the sole 5 of the cell. , by means of cam or ratchet mechanisms (dito, parag.4, 6). Vibrators such as 20, 21, 22, ..., 26 may all be identical or, alternatively, they may be of different power. In the latter case, the vibration zones of the hearth, as defined above, have

different lengths.

  According to the invention, during the operation of the cell, the vibrators are periodically actuated according to a well-defined operating cycle. For this purpose, the downstream vibrator 26 is first actuated, for example for a period of 15 to 30 seconds, the other vibrators being at a standstill, so that only the downstream zone 33 of the hearth is the seat of vibrations of sufficient intensity to detach the large mercury clusters that could adhere and allow their training by the mercury web. After stopping the downstream vibrator 26, the vibrator 25 which immediately precedes it is started, also for 30 seconds, so that this time the zone 32 of

  the sole 5 of the cell which is the seat of vibrations of inten-

  sufficient to detach any clumps of large mercury. Then proceed in an identical manner and step by step, for all the other vibrators 24, ..., 23, 22, 21, until

upstream vibrator 20.

  This way of proceeding results in all zones 33, 32, 31,

  30, 29, 28, 27 of the hearth 5 are vibrated successively and in order from the downstream end to the upstream end of the hearth. The large mercury clusters adhering to the sole 5 of the cell are in this way detached gradually and gradually from the downstream end to the upstream end of the hearth and discharged out of the cell by entrainment in the mercury layer 11, via the..DTD: conduit 12.

  At the end of the upstream downstream vibration cycle that comes

  to be described, we can start again an identical cycle of vibra-

  downstream upstream. The delay to be respected between stopping the upstream vibrator 20 at the end of a cycle and starting the downstream vibrator 26 at the beginning of the following cycle depends in particular on the length of the cell, the number of vibrators, the overall duration of a vibration cycle and the rate of formation of large mercury clusters on the hearth. Depending on the case, it can vary from a few minutes to several hours. Alternatively, in the critical cases of cells which are the seat of a large formation of large mercury clusters, it may sometimes be necessary to start a new cycle of downstream vibrations in

  upstream, before the previous cycle is completed.

  According to a preferred variant of the process which has just been described with reference to the figures, at the end of the vibration cycle from the downstream upstream of the hearth 5, a vibration cycle is started from upstream to 'downstream; for this purpose, after having actuated separately and successively the vibrators 26, 25, 24,

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  9 9- 23, 22, 21 and 20, it operates separately and in the order

  next, the vibrators 20, 21, 22, 23, ..., 24, 25 and then 26.

  This variant of the invention ensures optimum cleaning of the sole 5 of the cell and a regular and optimum evacuation of the large mercury clusters out of the cell, while reducing to a minimum value the disturbances in the flow of the water table. mercury and, consequently, in the course of the electrolysis. All other things being equal, it allows by

  in addition, minimum anode-cathode distances.

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Claims (7)

R E V E N D I C A T I 0 N S   1 - A process for the electrolysis of an aqueous solution of alkali metal halide in an electrolysis cell, the cathode of which is a flow sheet in flow on an inclined floor of the cell, in which the sole is vibrated, characterized in this   that separate areas of the sole are vibrated separately.   2 - Process according to claim 1, characterized in that the areas of the sole, which is vibrated separately, are contiguous. 3 - Process according to claim 1 or 2, characterized in that   that the distinct zones of the sole, which are vibrated separately,   are arranged one behind the other between the end   upstream and the downstream end of the mercury web.   4 - Process according to claim 3, characterized in that   that the areas of the sole are vibrated successively.   Process according to claim 4, characterized in that the zones are vibrated successively from the end   downstream to the upstream end of the mercury web.   6 - Process according to claim 5, characterized in that after having vibrated the areas of the hearth successively from the downstream end to the upstream end of the mercury layer, they are vibrated successively from the end   upstream to the downstream end of the mercury web.   7 - Process according to any one of claims 1 to 6,   characterized in that the zones of the sole are vibrated at a   frequency between 150 and 300 Hz.   8 - Process according to any one of claims 1 to 7,   characterized in that the zones of the hearth are vibrated by developing vertical forces of between 20 000 and 75 000 N.   9 - Process according to any one of claims 1 to 8,   characterized in that each zone is vibrated for a period of time between 15 and 30 s. - he -   - Process according to any one of claims 4 to   9, characterized in that between the vibration of a zone and the vibration of the next zone, a time interval equal to the time taken by the mercury layer is maintained to cover the length of the zone that has just been vibrate.