US20010013468A1

US20010013468A1 - System for adjusting the anode-cathode spacing in a mercury cathode electrolytic cell, by means of single-line frames

Info

Publication number: US20010013468A1
Application number: US09/774,666
Authority: US
Inventors: Roberto Prandoni
Original assignee: PRS DI ING ROBERTO PRANDONI & CS AS
Current assignee: PRS DI ING ROBERTO PRANDONI & CS AS
Priority date: 2000-02-16
Filing date: 2001-02-01
Publication date: 2001-08-16
Also published as: ITMI20000265A0; IT1316753B1; ITMI20000265A1

Abstract

A system for adjustment of the anode-cathode spacing in an electrolytic cell for the production of chlorine and soda comprises (FIG. 4) a liquid mercury electrode (2) disposed on the bottom (3) of the cell (100) and a plurality of anode electrodes (4) supported in transverse lines by mobile frames or subframes (11), adjustable in height, driven by means of respective lever systems (13) disposed between said groups of anodes (4) and an upper fixed frame (16) of the electrolytic cell (1), the lever system comprising a pair of levers (13) hinged at the fixed frame (16), the point of application of resistance thereof being connected to a respective subframe (11) and the point of application of force being connected to means of application of force (19, 20) supported by the fixed frame (16).

Description

DESCRIPTION

The present invention relates to a system for adjusting the anode-cathode spacing in mercury cathode electrolytic cells, particularly used for the production of chlorine and soda.

The production of chlorine and soda is achieved through electrolysis of a saturated solution of sodium chloride in electrolytic cells of various types. Among such electrolytic cells the mercury cathode cell is of great importance.

By way of example, FIG. 1 shows a typical mercury cathode electrolytic cell according to the prior art, denoted as a whole by

reference numeral

200.

The

cell

200 comprises an iron tank 1 on the bottom 3 of which mercury 2 flows, which forms the cathode. The bottom 3 is connected by means of copper bars 28 to the negative pole of a direct current power source.

The anode of the

cell

200 consists of a plurality of activated titanium electrodes 4, supported by one or more frames 25 that are moved manually or by means of drive systems controlled by processors.

The electrical current is carried to the

anode

4 by means of copper bars 5, called “current-carrying lines”, connected to the positive pole of a direct current power source. As shown in FIGS. 2 and 3, each current carrying line 5 supplies the current to a certain group of anodes 4, in the example three anodes. Each frame 25 supports a plurality of current-carrying lines 5 and respective groups of anodes 4, in the example four current carrying lines 5 each connected to a group of three anodes 4.

The tank (FIG. 1) of the cell is fed with brine saturated with sodium chloride which, as the current passes, decomposes and forms gaseous chlorine at the

anode

4 and an amalgam of sodium and mercury at the cathode 2. The amalgam of sodium and mercury leaves the tank 1, thanks to its sloping bottom 3, and enters a reactor 26, called a decomposer, filled with graphite and fed with water. On contact with the graphite, the amalgam of sodium and mercury, reacts with the water and forms hydrogen, soda and mercury; the latter is returned to the tank 1 of the electrolytic cell by the action of a recirculating pump 27.

The

mercury

2 flowing on the bottom 3 of the tank 1 can vary in thickness either due to problems in the recirculation pump or because of the deposits that form on the bottom 3 during the electrolysis process on account of the impurities contained in the brine. These phenomena, in relation to the processing conditions, can increase the thickness of the mercury cathode 2 from 1 to 20 mm, and in addition can also cause continuous variations in the anode-cathode spacing, of a local or general nature, because of the undulations that are formed on the surface of the flowing mercury.

The power consumption of the

electrolytic cell

200 is proportional to the inter-electrode distance between the cathode 2 and the anode 4. This distance is suitably maintained between 1 and 3 mm. On account of the phenomena described above, however, it is difficult to adjust said inter-electrode distance and moreover there is a likelihood of triggering short circuits between the anode 4 and the cathode 2, which destroy the electrodes 4 and create explosive situations inside the electrolytic cell 200.

For these reasons the

anodes

4 are supported by mobile frames operated manually or by microprocessors which control the anode-cathode spacing and adjust the position of the anodes 4 according to the variations in thickness of the mercury 2. Said control of the anode-cathode distance is indirectly achieved by monitoring both the anode-cathode voltage and the intensity of the electrical current in each current-carrying line 5. When the anode-cathode voltage or the intensity of the current in a current-carrying line 5 exceeds the preset limits, the frame 25 that supports that current-carrying line is controlled by the microprocessor to be raised or lowered in order to keep the preset anode-cathode spacing constant.

The maximum efficiency of adjustment systems of this type is obtained by adjusting anode surfaces which are as fractional as possible. Consequently

small frames

25 are used (FIG. 4) each having the smallest possible number of current carrying lines 5 and each current carrying line having the smallest possible number of anodes 4.

In practice, the optimal solution is obtained when each frame supports a group of anodes of a single current-carrying line and when the frame moving system is able to make movements from 0.1 to 0.2 mm. In fact it is known that voltages and power consumptions decrease more or less proportionally as the number of frames increases and as the number of current-carrying lines and the anode surface per frame decreases. For example, an optimal solution is that represented by frames comprising the anodes of a single current-carrying line.

With reference to the

electrolytic cell

200 and FIG. 2, it is obvious that it is easer to adjust/control the anode-cathode spacing by manoeuvring eight small independent frames 25 with a surface of about 1 m², instead of single frame 25 of about 8 m², especially in the presence of the above-described variations in the thickness of the mercury cathode 2.

Almost all plants with electrolytic cells were built in the 1960s and 1970s, when the technology of automatic adjustment systems for anode-cathode spacing was not adequate and the low cost of electric power did not justify investments to provide electrolytic cells with the necessary technology for accurate adjustment of the anode-cathode spacing. For these reasons the electrolytic cells according to the prior art, instead of providing a suitable subdivision of the anodes, provide large frames each comprising three to six current-carrying lines.

At present the technology provides computerized systems for adjustment and control of anode-cathode spacing which are very economical. However, the existing electrolytic cells are poorly suited to said adjustment systems because of the large size of the frames and the resulting difficulties in movement thereof. On the other hand, the systems for moving the frames according to the prior art are inaccurate and cannot be applied to smaller size frames.

A system for raising the frames of electrolytic cells according to the prior art uses four mechanical jacks disposed at the corners of the corresponding frames, and operated two by two by geared motors. This system is valid for its precision and operation, but can be used only for electrolytic cells with few, large-sized frames, in that it is extremely costly.

A lever system comprising four levers supported by four supports at the comers of a frame is also known to the art. Each pair of levers is moved by means of a geared motor. This system also is costly and difficult to adapt to small frames comprising one or two current carrying lines.

Also known to the art is a pulley system with pinion drive chains. Said system is inexpensive and lends itself to being mounted on small frames, but it introduces considerable play and wears easily, therefore it is inaccurate and little used.

Also known to the art is another system with a torsion bar which comprises a shaft that turns with angular shifting, mounted on an electrode carrying frame. With said system adjustment of the inter-electrode distance is inaccurate, since, during operation, the levers that permit movement of the frame change in length, introducing vertical shifts of the lever system that are not proportional to the vertical shifting of the frame. This system also is costly and can be used only for large frames.

A recently produced system, described in Italian patent application M197A 001434 in the name of the same applicant, achieves movement of the frame by means of four sliders that are moved along four inclined planes by means of two horizontal threaded shafts driven by two geared motors. Said system is suitable for medium and small size frames and is less costly than its predecessors, but it has problems of inertia, mechanical instability (the mobile frame can rotate along the opposite inclined planes) and of seizure both of the threaded shafts and of guides of the mobile frames. Moreover this system is not economical for installation on small frames comprising electrodes of a single current carrying line.

An object of the invention is to eliminate said drawbacks by providing a system for adjustment of the anode-cathode spacing for an electrolytic cell that is extremely precise, reliable, of simple construction and inexpensive.

Another object of the present invention is to provide such a system for adjustment of the anode-cathode spacing that can be installed on already existing frames of electrolytic cells.

Another object of the present invention is to provide such an adjustment system that is able to operate with automatic control systems.

These objects are achieved in accordance with the invention, as set forth in appended

independent claim

1.

Other advantageous embodiments of the invention are described in the dependent claims.

Essentially the system for adjustment of the anode-cathode spacing according to the invention comprises a plurality of small mobile frames, hereinunder referred to as subframes. Each subframe supports the anodes of a single current carrying line. Two levers fixed underneath the pre-existing frames and supported thereby are associated with each subframe. The levers are operated by a geared motor that causes raising or lowering of the subframe.

This adjustment system offers various advantages.

In fact the possibility of moving the subframes independently from each other, permits extremely precise adjustment of the anode-cathode spacing irrespective of the undulations of the mercury cathode.

Moreover the lever system for lowering and raising the subframes lends itself perfectly to the application of automatic control systems that can adjust anode-cathode spacing with extreme precision.

Lastly the system for adjustment of anode-cathode spacing according to the invention can be applied to pre-existing electrolytic cells without disturbing configuration thereof. In fact the frame of the pre-existing electrolytic cells, which acts as a fixed supporting frame for the adjustment system according to the invention, is maintained.

Further characteristics of the invention will be made clearer by the detailed description that follows, referring to purely exemplary and therefore non-limiting embodiments thereof, illustrated in the appended drawings, in which: [0031]
FIG. 1 is a diagrammatic view, in longitudinal section, of an electrolytic cell according to the prior art; [0032]
FIG. 2 is a top plan view of the electrolytic cell in FIG. 1; [0033]
FIG. 3 is a cross sectional view taken along the plane III-III in FIG. 2; [0034]
FIG. 4 is a cross sectional view of an electrolytic cell, provided with a system for adjustment of the anode-cathode spacing according to a first embodiment of the invention; [0035]
FIG. 5 is a top plan view of a portion of the electrolytic cell in FIG. 3; [0036]
FIG. 6 is a front view showing in detail only the system for adjustment of the anode-cathode spacing, according the first embodiment of the invention; [0037]
FIG. 7 is a top plan view of the adjustment system of FIG. 6; [0038]
FIG. 8 is side view of the adjustment system of FIG. 6; [0039]
FIG. 9 is a diagrammatic, axonometric view showing the system for adjustment of anode-cathode spacing according the first embodiment of the invention; [0040]
FIG. 10 is a diagrammatic, axonometric view of a detail of FIG. 9 showing a lever for movement of the subframe; [0041]
FIG. 11 is a diagrammatic, axonometric view showing the lever for movement of the subframe, according to a second embodiment of the invention. [0042]
A first embodiment of the system for adjustment of the anode-cathode spacing of an electrolytic cell in accordance with the invention is described with the aid of FIGS. [0043] 4-10. In this first embodiment the same numbers are used to indicate similar or corresponding elements to those described previously with reference to FIGS. 1-3 illustrating the electrolytic cell 200 according to the prior art.
With reference to FIG. 4, an [0044] electrolytic cell 100 comprises an iron tank 1, on the bottom 3 of which is a mercury cathode 2. A plurality of titanium anodes 4 are arranged at a short distance from mercury cathode 2.
The [0045] electrolytic cell 100 comprises various transverse rows of anodes 4. In the present embodiment, as shown in FIG. 4, each transverse row comprises three anodes 4. The anodes 4 are supported by respective copper pins 41 for carrying the current, which hereinunder for simplicity's sake will be identified with the anodes themselves.
The [0046] pins 41 of each transverse row of anodes 4 are connected by means of conducting or flexible elements 6 to a copper bar 5 which, as explained earlier, is called the current-carrying line. Each current-carrying line 5 is thus connected to three anodes 4 disposed on a transverse row.
The [0047] tank 1 is closed by a carpet 7 through which the pins 41 extend around which are disposed seals 8. The carpet 7 rests on side walls 9 and is secured by a section bar 10 disposed on said side walls 9.
The three [0048] anodes 4 of a transverse row are fixed, by means of steel tie rods 12 to a mobile frame 11, identified hereinunder by the name of subframe. The subframe 11 is supported by two levers 13 by means of four tie-rods 14 (see also FIGS. 8-10), disposed in the vicinity of the four corners of the subframe 11.
As better shown in FIG. 10, each [0049] lever 13 is substantially fork-shaped, formed by a single force arm 50 and two resistance arms 51. The force arm 50 is connected to the central part of a bar 53 disposed at right angles thereto. The two resistance arms 51 are connected to the two ends of the bar 53 and are parallel and opposite to the force arm 50.
Each tie-[0050] rod 14 is hinged to the end of a respective resistance arm 51 of the lever 13 by means of a pin 22 placed at the point of application of resistance.
Returning to FIG. 4, a fixed [0051] frame 16 is bolted by means of four bolts 24 to a main frame 25, pre-existing in all electrolytic cells of the prior art. The fixed frame 16 is arranged on mobile frame 11.
At a certain distance from the [0052] pins 22, the resistance arms 51 of each lever 13 are hinged, by means of pins 21, to two brackets 15 bolted to the fixed frame 16. The pins 21 form the points of fulcrum of the lever 13. At the end of the force arm 50, each lever 13 is hinged, by means of a pin 23, to a slider 17 that slides along a vertically disposed threaded shaft 18. The pin 23 forms the point of application of force.
In short, each [0053] lever 13 is a first class lever, in which the point of application of force is situated at the pin 23, at the end of the force arm 50; the point of application of resistance is situated along the axis passing through the two pins 22 at the ends of the two resistance arms 51 and the fulcrum is situated along the axis passing through the two pins 21 disposed in the two resistance arms 51 at a certain distance from the pins 22.
A [0054] motor 19 with a gear unit 20 that drives the threaded shaft 18 is fixed to the fixed frame 16. Operation of the motor 19 in one direction or the other, causes a rotary movement, clockwise or anticlockwise, of the threaded shaft 18 and a consequent raising or lowering of the slider 17. The slider 17, being hinged at respective points of application of force of the levers 13, causes raising or lowering of the respective force arms 50 of the levers 13; the resistance arms 51 of the two levers 13 are raised and lowered accordingly. Since the tie-rods 14 that support the subframe 11 are hinged at the points of application of resistance of the levers 13, lowering or raising thereof occurs, causing raising or lowering of the subframe 11 supporting the anodes 4.
The [0055] motor 19 and the gear unit 20 can be controlled by an automatic control logic, which, on the basis of the voltages detected between the anode 4 and the cathode 2 and the currents detected on the current carrying line 5, controls rotation in one direction or the other of the motor 19.
FIG. 11 shows a second embodiment of the invention, in which like or corresponding elements to those described in the first embodiment are denoted by the same reference numerals. [0056]
The only difference with respect to the first embodiment is that the two [0057] levers 13, instead of being first class levers are second class levers. As shown in FIG. 11, the lever 13 has the same configuration as that of the first embodiment. The lever 13 has a force arm 50 and two resistance arms 51.
As in the first embodiment, the [0058] force arm 50 is hinged at its end to the slider 17 by means of the pin 23.
In contrast with the first embodiment the two [0059] resistance arms 51 are hinged at their ends, by means of pins 21, to the brackets 15, in turn bolted to the fixed frame 16. Consequently the fulcrum of the lever is situated at the end of the resistance arms 51. The tie-rods 14, in turn integral with the subframe 11, are also hinged on the resistance arms 51, at a certain distance from the pins 21, by means of respective pins 22. Thus the point of application of the resistance of the lever is situated on the straight line joining the two pins 21, comprised between the fulcrum and the point of application of force, thus generating a second class lever.
In both embodiments mechanical safety stops (not shown) are provided to prevent movements of the [0060] subframe 11 beyond the minimum and maximum distance between the electrodes 4 and 2.
Various changes and modifications of detail within the reach of a person skilled in the art can be made to the present embodiments of the invention, without departing from the scope of the invention set forth in the appended claims. [0061]

Claims

1. A system for adjusting of the anode-cathode spacing in an electrolytic cell for the production of chlorine and soda, comprising a liquid mercury electrode (2) disposed on the bottom (3) of the cell (100) and a plurality of anode electrodes (4) supported in groups by respective mobile frames or subframes (11), adjustable in height, characterized in that said mobile frames (11) are moved by means of lever systems (13) disposed between said groups of anodes (4) and an upper fixed frame (16) of the electrolytic cell (1).

2. A system in accordance with

claim 1

, characterized in that said anode groups (4) supported by said mobile frames (11) are formed by transverse rows of anodes of the electrolytic cell (1), respectively connected to current-carrying lines (5).

3. A system according to

claim 1

or

2

, characterized in that said lever system (13) for movement of a respective mobile frame (11) comprises a pair of levers (13), each lever (13) being hinged to said fixed frame (16), having the point of application of resistance hinged to said mobile frame (11) and the point of application of force operationally connected to means for application of force (19, 20).

4. A system according to

claim 3

, characterized in that each lever (13) is fork-shaped, having a force arm (50) and two resistance arms (51).

5. A system according to

claim 4

, characterized in that the point of application of force is obtained by hinging each end of the force arms (50) of the two levers (13) to a slider (17) sliding vertically on a threaded shaft (18) that is moved by said means of application of force (19, 20).

6. A system according to

claim 5

, characterized in that said means for application of force comprise a motor (19) and a gear unit (20) controlled by a control logic to drive said threaded shaft (18).

7. A system according to any one of

claims 3

to

6

, characterized in that said means for application of force (19, 20) are supported by said fixed frame (16).

8. A system according to one of

claims 4

to

7

, characterized in that the fulcrum of each lever (13) and said point of application of resistance are situated on said resistance arms (51).

9. A system according to

claim 8

, characterized in that said mobile frame (11) is supported, in the vicinity of its four comers, by four tie-rods (14) hinged at the ends of the four resistance arms (51) of the pair of levers (13), so that the fulcrum of each lever is situated between the point of application of force and the point of application of resistance so as to have first class levers.

10. A system according to

claim 8

, characterized in that in the vicinity of the ends of the two resistance arms (51) of each lever (13), are hinged respectively two brackets (15) integral with the fixed frame (16), so that the point of application of resistance is situated between the point of application of force and the fulcrum, so as to have second class levers.

11. A system according to any one of the preceding claims, characterized in that said fixed frame (16) is integral with a pre-existing frame (25) in the supporting structure of the electrolytic cell (1).

12. An electrolytic cell for production of chlorine and soda, comprising a system for adjusting the anode-cathode spacing according to any one of the preceding claims.