IE51293B1 - Electrolytic cells - Google Patents

Abstract

To solve pollution problems in electrolytic cells for the production of zinc by the electrolysis of a solution of a zinc salt, shielding baffles (11) are provided above the cell anodes (7), the gap between adjoining baffles (11) being such as to provide a satisfactory tight seal while permitting the evacuation of noxious gases and mists through manifolds formed in the cell walls. The gaps between adjoining baffles (11) enable the cell cathodes (9) to be periodically removed from the cell for the removal of zinc therefrom.

Description

This invention relates to electrolytic cells, particularly to electrolytic cells for the production of zinc. The electrolytic cells of the invention can be used for carrying out various electrolytic processes in which gases are evolved, but are particularly suitable for carrying out processes for the electrolytic production of zinc. Herein, particular reference will be made to the production of zinc, it being understood that the cells of the invention can be used with advantage to effect other industrial electrolytic processes.

In a known process for producing primary zinc, a number of stages are carried out to obtain a high-purity acidic solution of a zinc salt, generally zinc sulphate, which solution is reduced in a final stage to metallic zinc by electrolysis. The electrolytic cells used in the process consist of vats in which there are numerous confrontingly positioned anodes and cathodes.

As a rule, the anodes are of lead and the cathodes are of aluminium.

Metallic zinc is deposited on the cathode to form a thin sheet, which is removed from time to time by stripping. The gaps between adjoining electrodes must be as small as practicable in order to limit the voltage drop through the solution (which voltage drop is proportional to the gap between the electrodes).

The electrodes must be parallel to each other to obtain an even deposit on the entire cathode surface, and thus a well formed sheet.

The primary and secondary reactions which take place in the electrochemical cell are as follows: ZnS04 + H2 -* Zn + H2SO4 + i 02 (1) 51393 (2) Zn + H2SO4 ZnS04 + H2 (3) The first two reactions take place concurrently, and the predominance of reaction (1), i.e. the primary reaction, over the reaction (2) determines the electrolytic yield of the cell. The redissolution reaction (3) may take place accidentally when the supply of electricity is discontinued. The reactions (2) and (3) may take place either due to chemical reasons, such as inadequate purification of the electrolyte, and/or for physical reasons, such as temperature.

An effect of the evolution of gas in the cell, be it under normal conditions, i.e. under conditions in which gaseous oxygen is produced according to reaction (1), or under abnormal conditions, i.e. under conditions in which hydrogen is produced according to reactions (2) and (3), is the formation of acidic mists. These mists are the result of the formation of tiny droplets of acidic solution which, especially under abnormal electrolysis conditions, are entrained from the electrolyte so as to become airborne.

Another undesirable effect is experienced when the working conditions are “abnormal. This effect is the formation of localised explosive mixtures.

Under normal conditions, the content of mists in the environment around an electrolysis cell is around a few milligrams per normal cubic metre of air, - 3 51293 measured in terms of sulphuric acidity. Under abnormal conditions, this value is raised to appreciably higher values.

The stripping of the zinc from the cathodes takes place at preselected time intervals, generally 24 to 28 hours, and is carried out mechanically. This operation requires the attendance of operators near to the cells so that the operators are exposed to the harmful environment. This exposure, in the long run, brings about alteration of the nasal sept, the oral cavity, the teeth and the respiratory tract in general. The environment, moreover, is harmful to nearby machinery and implements, and to the structure of the buildings in which the cells are housed, so that these must be such as to withstand the environment.

Attempts have been made to overcome these short-comings by purifying the work environment by forced ventilation, with a total renewal of air from 5 to 10 times per hour. This procedure, however, has a number of drawbacks, such as the production of strong air currents, the difficulty of air-conditioning, especially for installation in cold areas, and a considerable waste of power.

According to the present invention, there is provided an electrochemical cell comprising a vessel for containing electrolyte and a plurality of anodes and cathodes arranged in the vessel in alternating sequence, each anode having a baffle associated therewith which baffles extend over the vessel and form a cover for the vessel, adjacent baffles having therebetween a gap wherein a respective one of said cathodes is positioned and the vessel having manifold means for the withdrawal of gas generated during use of the cell, and of any electrolyte mist entrained in such gas, from the space between the cover and the electrolyte contained in the vessel during use of the cell.

For a better understanding of the invention, reference will now be made, by way of example to the accompanying drawings, in which: Figure IA is a cross-sectional view of the vessel of a cell of the invention; Figure 1B is a plan view of the vessel of Figure 1A; Figure 2 is a perspective view of the electrode assembly to be disposed in the vessel of Figures 1A and 1B; and Figure 3 is an exploded perspective view of the cell consisting of the vessel of Figures 1A and IB and of the assembly of Figure 2.

Figures 1A and 1B are diagrammatic cross-sectional and plan views, respectively, of a vessel of a cell of the invention. The vessel has suction manifolds 1 and ports 2 formed in an enlargement 3 of the top portion of the sidewall 4 of the vessel. The electrolyte level is shown at 5. As an alternative, the manifold can be provided on only one side of the vessel. The vessel has an overflow 6.

Referring to Figure 2, a set of anodes 7 and a set of cathodes 9 are immersed in the vessel in alternating sequence, and rest on supporting members provided on the top edge of the cell. The cell is covered by a set of shielding baffles 11, each secured along the entire length of a respective anode and cut out in correspondence with the centre line of the gaps between the anodes 7, i.e. in correspondence with the working positions of the cathodes 9. The empty space so provided, in addition to making it possible to remove and replace the cathodes 9 for stripping the deposited zinc layer, provides a reference which is useful for a correct positioning and an inmediate check of the electrode alignment. The electrode assembly includes bus bars 8 for the π- anodes 7, and bus bars 10 for the cathodes 9.

S12S3 The shielding baffles 11 are secured to the bars 8 by spring members 12 and/or by screws 13. The baffles 11 are preferably made of a resilient material so as to provide a satisfactory tightness while not hindering the removal of the cathodes. The baffles 11 can be fastened in ways other than as described above, such as by forming in the horizontal top surfaces of the bars 8 a set of grooves, the baffles 11 having projections matching such grooves.

The baffles 11 define a space above the electrolyte, and the gases which are evolved during electrolysis, together with the acidic mists entrained therein, are withdrawn from this space. Thus, the suction manifolds 1 are connected to a conventional suction system equipped with electric exhausters and/or ejectors.

Figure 3 is an exploded perspective view of the cell, consisting of the vessel of Figures IA and IB, the electrode assembly of Figure 2, and of a support assembly for supporting the electrode assembly in the vessel.

The support assembly consists of two square cross bars upon which the anodes and cathodes rest, and two side plates having projecting lugs which rest upon the vessel. The support assembly, or at least the cross bars thereof, is made of a non-conductive material. A handling device, detached from the supporting device, is also shown.

The electrolytic cell described above with reference to the Figures enables the aforementioned drawbacks to be overcome since the withdrawal of the acidic mists is effected by the removal of small volumes of air and thus with a small consumption of power, so that air-conditioning of the room in which the cell is housed is not a problem. In addition, the cell permits one to recover, if desired, an oxygen-enriched atmosphere. The withdrawn gases and mists can be dispersed into the atmosphere through a chimney in a diluted state so that they are harmless. As an alternative, a gas-abating system can be provided, to be used at least when abnormal reaction conditions are encountered.

If electric exhausters are used, those in which the moving members have antiexplosion bearings are preferred.

The negative pressure used for withdrawing the air above the electrolyte is usually up to 15 mm of water column. This value is dependent upon the sealtightness of the baffles, and too low absolute values of the negative pressure do not afford adequate exhaustion, whereas too high values cause excessive air to be drawn through the suction gaps, which air dilutes the gases without any advantage while requiring a higher power consumption. If a system for abating the withdrawn gases is provided, the suction system is subjected to additional pressure drops. The following Table gives working data obtained for a test cell of the invention. TABLE Composition of the atmosphere within the cell Operational conditions Increased 02 content (% by volume) H2 content (mg/Nm3) H2S0, content (mg/ftm3) Normal + 1.0 300 1.0 Normal + 2.0 250 0.8 Abnormal + 2.0 550 3.0 Abnormal + 1.2 700 7.0 Abnormal + 2.0 1000 15.0 Abnormal + 3.0 400 5.0 By maintaining a suction of 150 Nm3/hour in the suction manifolds 1, no detectable alterations of the composition of the air in the cell room have been experienced. - 7 51293 The cells according to the invention can be made of conventional materials such as coated or protected cement, resistant plastics materials (e.g. reinforced polyvinyl chloride) and resin-impregnated glass fibre. Air ports, manifolds and suction fittings can be formed in the vessel of the cell during its construction.

Alternatively, they can be formed subsequent to the construction of the vessel. The vessel may be provided with ribs for increasing the sturdiness and the stiffness of the vessel, and in this case the manifolds and ports may be formed in the ribs. The manifolds can also form a supporting structure for the electrodes.

A possible alternative to the suction system with negative pressures is that of pressurizing the cell environment to a value slightly above atmospheric so as to prevent the flow of gas from the space above the electrolyte level to the environment, but in such a way that reverse flow from the cell environment to the space above the electrolyte is prevented and thus the flow towards the manifolds is also prevented. Such an approach, which is doubtless more intricate, might be more prudential and thus preferred for particular electrochemical processes in the electroplating industry.

Claims (8)

1. 20 1. An electrochemical cell comprising a vessel for containing electrolyte and a plurality of anodes and cathodes arranged in the vessel in alternating sequences, each anode having a baffle associated therewith which baffles extend over the vessel and form a cover for the vessel, adjacent baffles having therebetween a gap wherein a respective one of said cathodes is positioned, and 25 the vessel having manifold means for the withdrawal of gas generated during use of the cell, and of any electrolyte mist entrained in such gas, from the space between the cover and the electrolyte contained in the vessel during use of the cell. 51883

2. A cell as claimed in Claim 1, wherein the baffles are made of a resilient material.

3. A cell as claimed in Claim 1 or 2, wherein each anode and its associated baffle are secured to each other so that the cathodes can be 5 removed from the cell without removing the baffles.

4. A cell as claimed in any of Claims 1 to 3, wherein the manifold means comprises a manifold formed in a sidewall of the vessel, and a plurality of ports each connecting the manifold to said space between the cover and the electrolyte. 10

5. A cell as claimed in any of Claims 1 to 4, being a cell for the electrolytic production of zinc.

6. A cell as claimed in any of Claims 1 to 5, when the manifold means is connected to a suction means for the withdrawal of gas and any electrolytic mist entrained therein from said space between the cover and the electrolyte. 15

7. A cell as claimed in any of Claims 1 to 6, when disposed in an environment having a pressure slightly exceeding that in said space between the cover and the electrolyte.

8. An electrochemical cell as claimed in Claim 1, substantially as hereinbefore described with reference to, and as shown in, the 20 accompanying drawings.