WO2008142632A2

WO2008142632A2 - Electrolytic cell and electrolyzing equipment

Info

Publication number: WO2008142632A2
Application number: PCT/IB2008/051957
Authority: WO
Inventors: Elio Mario Boschetti
Original assignee: Inur S.A.
Priority date: 2007-05-23
Filing date: 2008-05-19
Publication date: 2008-11-27
Also published as: WO2008142632A3; BRPI0701653A2

Abstract

The present invention relates to an electrolytic cell, particularly designed for obtaining caustic soda (NaOH) and halogens by electrolysis, provided with at least one first electric terminal and at least one second electric terminal positioned adjacent each other for carrying out the electrolysis, characterized in that the first electric terminal is an anode, centrally positioned in the electrolytic cell, constituting an anodic chamber (20) and the second electric terminal comprises two cathodes, each positioned laterally respective to the anode and opposite each other and constituting a respective cahodic chamber (30), the anode and each cathode being separated by means of at least one semi-permeable membrane (4).

Description

Electrolytic cell and electrolyzing equipment

The present invention relates to electrolyzing equipment, particularly designed for the production of caustic soda, as well as to an electrolytic cell that forms the equipment.

Description of the Prior Art

The electrolysis for obtaining caustic soda NaOH and halogens is a process known and widespread since long ago, being carried out inside electrolytic cells.

Caustic soda is obtained by electrolysis from demineralized water and brine (which should be virtually free from calcium and magnesium, besides other chemical elements the contents of which have to be quite low, for which reason being treated in ion-exchange columns). Electric energy is used in the process.

Electrolysis of sodium chloride (NaCl) present in brine is carried out in electrolysis cells, known also as electrolytic cells. Generally speaking, an electrolytic cell comprises two compartments (one with the anode and the other with the cathode), separated by a membrane.

The chemical reaction that takes place inside the electrolytic cell is the following:

2NaCl + 2H₂O -> Cl₂ + H₂ + 2NaOH

Brine is introduced in the anodic compartment and, with application of electric current; the bindings of a part of the NaCl molecule are broken, giving rise to sodium ions (Na⁺) and Chlorine ions (Cl^"). The resulting brine, which is depleted, comes out of the electrolytic cell.

The chlorine ions (Cl^") are attracted to the anode due to their negative electric charge and loose one electron, given rise to the formation of chlorine gas Cl₂ (anodic reaction). The sodium ions (Na⁺) are attracted to the cathode due to their positive electric charge, passing through the selective semi-permeable membrane that allows positive ions to pass, but prevents negative ions from passing.

NaOH diluted in water is introduced in the cathodic compartment. The water in the cathode zone forms H⁺ and OH^" ions. H⁺ ions, due to their electric charge, are attracted to the cathode where they receive an electron and form hydrogen gas molecules H₂ (cathodic reaction).

OH^" ions, in turn, are attracted to the anode and in this movement react with the sodium ions Na⁺ that pass through the membrane, giving rise to caustic soda, in a 32% solution.

In short, in an electrolytic cell for producing caustic soda, one feeds concentrated brine and demineralized water, and the compounds obtained after the electrolysis are NaOH 32%, chlorine gas and hydrogen gas, besides the depleted brine.

Even though this basic process is known and used since long ago, various improvements have been developed with a view to increase the efficiency of the caustic soda process, as well as to minimize potential damages to the environment by contamination, as for in- stance, the escape of chlorine gas.

When a plurality of electrolytic cells is used in conjunction, for increasing the production of caustic soda, one can carry out two types of connection between them.

The first type of connection configures the so-called bipolar electrode system, in which a plurality of electrolytic cells is electrically connected in series (cathode - anode -cathode - anode, etc). With the connection in series, little electric current is required for the production of caustic soda, and the distribution of the current is equal in all the cells, even though the voltage should be high.

A second type of connection configures the monopolar electrode system, where the cells are electrically connected in parallel, which reduces the necessary voltage of the current applied, but in return makes the increase of its amperage imperious. As disadvantages, in addition to a greater demand for electric current, non-uniform distribution of the electric current between the cells may occur.

In the case of the bipolar electrode system, a noteworthy improvement is disclosed in US Pat. 5,015,354, which disclosed a bipolar system in which there is a considerable reduction in the number of necessary compartments.

In a conventional bipolar electrode system having five cells interconnected in series, there are five cathodic compartments and five anodic compartments, in a total of ten necessary compartments. Such a number of compartments bring drawbacks for the installation, such as a large amount of tubing necessary to feed the system and to remove the electrolysis products.

This US patent proposes the removal of the diaphragms or walls of conventional cells and the positioning of two electrodes of the same polarity within the same compartment, which reduces by half the number thereof. Such a construction generates great simplifications, chiefly as far as the necessary tubing is concerned.

US patent US 3,883,415 proposes a bi-polar electrolytic cell system, in which each cell comprises a set of two titanium plates centrally positioned in the vertical, and a cathode in the form of a box or structure configured so as to enclose the anodic titanium plates. In this way, a uniform distance can be maintained between the anodic plates and the cathode, optimizing the electric current required for carrying out the electrolytic reactions therein.

US Patent 6,224,720 proposes an improved electrolytic cell having a plurality of bipolar electrodes (connected in series), where each of the bipolar electrodes has a cathodic portion and an anodic portion, between which an electrically conductive connection takes place during operation of the cell. At least one of the bipolar electrodes has a cathodic portion and an anotic portion that are designed to be moved relative to each other.

With a view to increase the productivity of caustic soda, patent application US 2004/0238351 discloses an electrolytic cell for obtaining caustic soda, which comprises an auxiliary module of the same geometry as the main module, containing interconnected cathodes and anodes, this auxiliary module being hydraulically movable from a rest position to an operation position, being connected in parallel to the main module.

Objectives of the Invention

The present invention has the objective of providing an electrolytic cell, particularly designed for the production of caustic soda by electrolysis, which is connected to other simi- lar electrolytic cells in series, obtaining the production of soda with efficiency and reduced electricity consumption, besides being extremely desirable, since it enables the production of caustic soda in small quantities with efficiency and competitive costs, for local (in situ) production by those that use this product.

The present invention also has the objective of providing electrolyzing equipment for the production of caustic soda, which comprises a number of pretended electrolytic cells, so as to achieve an excellent productivity with low cost of electricity and high useful, besides being extremely desirable for enabling the production of caustic soda in small amounts and with competitive costs for local (in situ) production by those that use this product.

Brief Description of the Invention

The objectives of the present invention are achieved by means of an electrolytic cell, particularly designed for obtaining caustic soda (NaOH) by electrolysis, having at least one first electric terminal and at least one second electric terminal positioned adjacent each other for carrying out the electrolysis, wherein the first electric terminal is an anode positioned centrally in the electrolytic cell, constituting an anodic chamber, and the second electric terminal comprises two cathodes, each positioned laterally with respect to the anode and opposite each other and constituting a respective cathodic chamber, the anode and each cathode being separated by at least one semi-permeable membrane.

Also, the objectives of the present invention are achieved by means of electrolyzing equipment, particularly an equipment designed for the production of caustic soda (NaOH) by electrolysis, having at least one body portion that defines an inner cavity for positioning and operating at least one electrolytic cell, the equipment further comprising:

(i) an electrolytic cell having at least one first electric terminal in the form of an anode positioned centrally, constituting an anodic chamber and at least one second electric terminal in the form of two cathodes, each positioned laterally with respect to the anode and opposite each other and constituting a respective cathodic chamber, the anode and each cathode being separated by at least one semi-permeable membrane; and

(ii) at least one integrated recirculation system for enabling the electrolytic cell to operate, which:

(11.1) operates by difference in temperature and has at least two incorporated heat exchangers;

(11.2) operates by difference in density between fluid columns; enabling preheating and maintenance of operation temperature of the equipment and uniformity in the concentration of the electrolytic fluid in the anodic and cathodic chambers.

The present invention presents, as advantages, the possibility of pre-heating and maintaining the operation temperature of the equipment (at about 85°C) and the uniformity in the concentration of the electrolytic fluid in the anodic and cathodic chambers of the electro- lytic cells, besides enabling the production of caustic soda with efficiency and competitive costs, for local (in situ) production by those that use this product.

Brief Description of the Drawings

The present invention will now be described in greater detail with reference to an embodiment represented in the drawings. The figures show:

- Figure 1 is a schematic top view in section of the electrolyzing equipment of the present invention;

- Figure 2 is a schematic side view in section of the equipment illustrated in Figure 1;

- Figure 3 is a schematic front view in section of the equipment illustrated in Figures l and 2; - Figure 4 is an exploded schematic view of an electrolytic cell that integrates the equipment illustrated in Figures 1 to 3;

- Figure 5 is an exploded schematic view of an electrolytic cell illustrated in Figure 4; - Figure 6 is an exploded schematic view of the electric connection in series of the electrolytic cells in the equipment illustrated in Figures 1 to 3;

- Figure 7 is a non-exploded schematic view of the electric connection in series of electrolytic cells in the equipment illustrated in Figures 1 to 3;

- Figure 8 is a schematic side view of the anode of the electrolytic cell illustrated in Figures 3 and 4;

- Figure 9 is a schematic front view of the anode of the electrolytic cell illustrated in Figures 3 and 4;

- Figure 10 is a schematic side view of the cathode of the electrolytic cell illustrated in Figures 3 and 4; - Figure 11 is a schematic front view of the cathode of the electrolytic cell illustrated in Figures 3 and 4.

Detailed Description of the Figures

According to a preferred embodiment and as can be seen from Figure 1, the electrolytic cell 1, especially designed for the production of caustic soda (NaOH), is used in electrolyz- ing equipment, which operates with high productivity, since it has a plurality of cells 1 electrically associated in series.

Preferably, the electrolytic cell 1 is small and designed for the production of caustic soda with efficiency and economy, and its preferred dimensions are 0.65 m² (1040 millimeters (mm) x 440 mm x 110 mm). In studies carried out, one has come to the above dimensions, wherein the cell width is smaller than its height. In other words, the shape of the cell is substantially that of a rectangular prism.

Thus, a minimum of material in manufacturing the cell is employed without the resulting cell ceasing to exhibit good electric, mechanical and electrochemical construction. With this design, it is not necessary to provide a large amount of titanium, which considerably reduces the manufacture costs, since this material is known to be expensive.

The electrolytic cell 1 aims at the production of caustic soda efficiently and with reduced electricity consumption, besides being extremely desirable since it enables the production in small quantities and with efficiency and competitive costs, for local (in situ) production by those that use this product.

The electrolytic cell 1 is an innovation and, essentially, comprises at least one first electric terminal 2 and at least one second electric terminal 3 positioned adjacent for carrying out electrolysis.

The first terminal 2 is an anode (has positive electric charge (+)) and has a substantially rectangular prism shape, it height and length being substantially greater than its width, de- fining a structure that has two main side walls of large dimensions, the area of which corresponds to the product of its height multiplied by its width, constituting/delimiting an inner chamber 20 called anodic chamber. However, it is evident that the shape of the anode 2 may vary, if necessary or desirable. The anode 2 is positioned centrally in the electrolytic cell, and the two main side walls are preferably made of activated titanium, an anodic tita- nium structure 21 being also provided, which preferably is substantially U-shaped and surrounds the walls of the anode 2, and it is through the titanium piece 21 that the electric current flows in the anode.

The U-shaped structure 21 also acts maintaining the side walls correctly positioned in the anode when pressure is exerted on them, mainly during the functioning of the cell 1.

The side walls of the anode 2 may further comprise openings of varying number, shape and positioning, to enable correct flow of electrolytic fluid (which will be mentioned later). Bores are provided so as to enable the passage of the electrolytic fluid and of the gases in both directions, preventing the formation of gas bubbles.

The second electric terminal 3 is negatively (-) electrically charged and preferably corre- sponds to two cathodes, each positioned laterally with respect to the anode 2 and opposite each other. Preferably, it is manufactured from stainless steel 316L.

Describing in greater detail, each cathode 3 is parallel and adjacent each of the two main side surfaces of the anode 2 in the form of rectangular prism. Just as the anode 2, each cathode 3 has also a substantially rectangular prism shape, with height and length substantially larger than the width, defining a structure that has two main side walls of large dimensions, the are of which corresponding to the product of the height of the cathode multiplied by its width. Each cathode 3 also constitutes/delimits an inner chamber 30 called ca- thodic chamber.

It is evident, however, that the shape of the cathodes 3 may vary, if necessary or desirable, provided that they cooperates with the anode 2 so as to enable correct assembly and operation of the cell. 1

The cathodes 3 are associated to the anode 2 so that one of the side walls of each cathode 3 will be parallel and adjacent one of the walls of the anode 2, in such a constitution that the anode 2 will be "sandwiched" between the two cathodes 3. Each cathode 3 is separated from the anode 2 by at least one semi-permeable membrane 4, which enables selective pas- sage of ions resulting from the electrolysis, which will be discussed later. The cathodes 3 are preferably formed from stainless steel.

The membrane 4 is preferably made from a material called Nafion, from the company Du Pont, but it is evident that it may be constituted by any other material that can operates satisfactorily.

Like what was said with regard to the anode 2, the side wall of each cathode 3 that is adjacent with the anode may comprise openings varying in number, shape and positioning, so as to enable correct flow of the electrolytic fluid.

In order for the electrolytic cell 1 to operate, with chemical reactions taking place in the presence of electricity (electrolysis), an electrolytic fluid comprising chemical components that undergo chemical reactions due do the electric current passing through it has to circulate inside said cell.

When the electrolytic fluid is inside the cell 1, the electricity flows through it, interfering with the connection between the constituent atoms, causing transformation of determined chemical substances into others. Specifically, the electric current for the electrolysis goes into the cell 1 through the outer sides of the anode 2 and comes out through the opposite sides of the cathodes 3, after passing through the electrolytic fluid and the semi-permeable membrane 4, whereby the anode 2 and the cathodes 3 have respective copper connections 2', 3', connected to the supply of electricity (in the case of the electrolysis equipment being described now, the cells 1 are connected in series, which will be described later) and suitable for the purpose.

Further with regard to the electrolytic cell 1 , it should be noted that it has at least one open- ing for entry of electrolyte, an opening for the entry of caustic soda diluted in water and at least one outlet for post-electrolysis fluid (depleted electrolyte) and products from the reaction, which in this case are not caustic soda (NaOH) in strong solution, chlorine gas and hydrogen gas. Preferably, the cell 1 comprises inlet for electrolyte, two inlets for caustic soda diluted in water and three outlets, one for the depleted electrolyte, two for the caustic soda (in strong solution). Preferably, there are three outlets, two for caustic soda in solution and hydrogen gas and the other for depleted brine and chlorine gas.

For the entrance of electrolytic fluid in the cathode 3, entrance tubing 31 are provided, which has a baffle 33 for giving a determined direction to the liquid flow. This direction, determined by the baffle 33, causes the velocity of the fluid to produce in the cathodic chambers 30 such a flow direction that the result is the rapid detachment of the hydrogen gas produced / obtained therein and an internal recirculation of the flow in upward direction against the semi-permeable membrane 4 and in downward direction against the wall of the corresponding cathodic chamber 30.

Even though the anode 2 and the cathodes 3 are mounted adjacent each other, they are electrically separated by insulating materials, so that the electric current should necessarily flow through the electrolyte. Cathodes and anodes are connected to a source of electric energy (DDP), which may have varying values of nominal voltage and amperage, as necessary or desired (the preferred values will be mentioned later).

The openings for entrance and exit are generally illustrated in the figures, as at least one first opening 5 provided centrally in the cell 1 for the entrance of products and at least one second opening 6 provided centrally in the cell 1 for the exit of fluids resulting from the electrolysis.

Figures 3 and 4 illustrate schematic partial views in section of the electrolytic cell of the present invention 1, where one can clearly see the copper terminals 2', which are superimposed over the surface of the titanium anode structure 21, forming the mark of the anode 2 proper, and that is associated to the side walls, preferably by means of soldering.

The electric current is transmitted from each terminal 2' to the structure 21 and from the latter to the side walls of the anode 2. After this, the electric current passes through the electrolytic fluid and the membrane 4, penetrating into the cathode 3 through its main side walls made of stainless steel, preferably perforated. From there the electric current passes through the Z-shaped pieces 32 that configure electric current conductors between the ac- tive cathodic surface and the cathodic box. From the cathode the electric current flows out through the copper terminal 3'.

The relation between the dimensions of the cell 1 (mentioned before), the structural characteristics of the electrodes and the density of the electric current are such that there is a uni- form volume of gas exit and depleted brine inside the anode 2, preventing the gases from accumulating in the upper portion of the anodic chamber, which could cause variation in the functioning pressure inside the cell and might damage the membrane 4. Figures 8 and 9 enable one to view the anode 2 in schematic longitudinal and transverse section, where one can see the U-shaped anodic structure 21, the terminals 2', the soldering (S) between the U-shaped anodic structure and the side walls.

Figures 10 and 11 enable one to view the cathode 2 in schematic longitudinal and trans- verse section, where the active surface of the cathode is said plate of stainless steel with bores or perforations that enable free circulation of the hydrogen gas and of the caustic soda in solution produced inside the cathodic chamber 30.

One can further observe Z-shaped structural elements 34 that, besides fixing the cathode in its correct position, enable variation of the position of the electrode at the moment of its construction, enabling one to adjust the measure of separation between them. Additionally, the Z-shaped elements 34 are electric current conductors, conducting it from the active surface of the cathode.

The same figures further enable one to view the internal part of the cathode 3, showing details of the side walls that have grooves to enable passage of fluid and gas between them.

The already mentioned baffling plate, or baffle 33 directs the incoming electrolyte fluid produced, together with the gas released from the electrolysis, a rapid upward stream against the membrane 4. This upward stream evidently has as a counterpart the downward stream of the same electrolyte flow, generating a rapid internal circulation, which guarantees the uniformity of the concentration of the electrolytic fluid inside the cell 1.

A preferred example of an electrolytic cell constituted with the teachings of the present invention (not limiting) has the following characteristics:

• It carried out electrolysis of an aqueous solution of 0.3 kg of NaCl per liter of solution;

• The semi-permeable membrane 4 used is Nation, produced by the company Du- Pont;

• 14 cells are electrically connected in series; the inlets and outlets of fluids of all of them are connected in parallel;

• a constant electric current of 2400 Amperes (A) is applied for a period of 12 months, obtaining a voltage per cell of about 3.8 volts (V), which is ideal as work- ing charge;

• as an electrolysis product, in each cell it is obtained 3.05 K/h of chlorine gas on the anodic surface, 0.08 K/g of hydrogen gas on the cathodic surface and 3.44 K/h of caustic soda at 32% (high purity). The output of the cell is of about 96%, similar to that obtained by bigger cells of industrial level.

An electrolytic cell having been described, one will now describe in greater detail a piece of electrolyzing equipment 100, particularly designed for the production of caustic soda (NaOH) by electrolysis, which comprises at least one electrolytic cell 1 defined before. Indeed, preferably the equipment 100 comprises a plurality of cells 1 electrically associated in series, with a view to maximize the production of caustic soda. More preferably, the equipment 100 comprises fourteen cells 1 electrically connected in series.

Structurally, the equipment 100 comprises at least one body portion 200, which defines an internal cavity for positioning and operating the fourteen electrolytic cells 1 , preferably but not compulsorily from carbon steel.

The body portion 200 further comprises at least one first tubing 201 for feeding electrolyte to the cells, a second tubing 202 for the entrance of caustic soda diluted in water, a third tubing 204 for exhausting depleted electrolyte and chlorine gas, and a fourth tubing 203 for the exit of caustic soda in strong solution and hydrogen gas.

The positioning of the fourth pieces of tubing may vary greatly according to the needs of project of the equipment 1, but preferably the first tubing 201 and the second tubing 202 are located in the lower portion of the cell 1, while the pieces of tubing 203 and 204 are located in the upper portion of the equipment 100. A configuration of the equipment 100 preferably has a single tubing with the function of exhausting the hydrogen gas and the caustic soda in strong solution and another single tubing with the function of exhausting the depleted electrolyte and the chlorine gas.

As innovations, besides the cells 1 themselves that constitute the electrolyzing equipment 100, the latter comprises at least one integrated recirculation system to enable the operation of the electrolytic cells, which per se is innovatory.

The integrated recirculation system operates by difference in temperature, for which reason it has at least one, but preferably two, heat exchangers 300 incorporated therein, beside being able to operate by difference in density between columns of fluid.

By virtue of the integrated recirculation system, one can pre-heat and keep the equipment operation temperature (about 85°C) and the uniformity in the concentration of electrolytic fluid in the anodic and cathodic chambers 20, 30 of each cell 1. The difference in density between the columns of liquid, which may vary depending on the respective temperatures, enables the uniformity in the concentration of the electrolyte present in the anodic 20 and cathodic 30 chambers.

Preferably, the heat exchanger 300 is associated to the body portion 200, and there are means for injecting cold or heated water into this exchanger 300, for the purpose of changing the temperature of the electrolytic fluid that circulates inside it. More preferably, two heat exchangers 300 are provided, which can assume any particular constitution, if neces- sary or desirable.

As can be seen from Figure 3, the equipment 100 comprises, positioned above, said pieces of tubing 203, 204, the pieces of tubing 204 and 205 being constituted by PRFV and having a liquid-gas separator 207 at their ends. The connection of each cell 1 with the connect- ing tubing is preferably made by means of translucent Teflon 211, joined by means of suitable clamps, but it is evident that other materials may be used.

In this figure, one can further see the inlet for water 208 to enable alteration in the concentration of NaOH by means of a tube of reduced thickness located in the recirculation flow that passes through the heat exchangers. The heat exchanger itself is the place where the alteration of the concentration will take place, by mixture between the brine and the fed water.

The recirculation connection through the heat exchangers 300 enables regulation/alteration of the cell operation temperature.

In the event that the temperature is low, which occurs specially in the pre-heating period or with low electric current charge, heated water into the heat exchangers 300 is introduced, which heats the electrolytic fluid in the desired measure.

On the other hand, when the cells operate above the ideal operation temperature (85⁰C), which occurs when the cells operate with excess current or if the equipment 100 is located at very hot places, cold water is introduced into the heat exchangers 300.

The equipment further comprises draining tubes 209 and 210, which serve to empty the equipment 100, as when in maintenance procedures, and a further level tube 211 is pro- vided, which is made from translucent Teflon 12 and which enables one to check the level of the electrolytic fluid in the cells 1.

This checking is important, especially before the equipment 100 is put in operation, since it is important to maintain negative pressure inside the anodic chamber 20 relative to the ca- thodic chambers 30 of each cell 1, in order to keep the membrane 4 correctly positioned.

The fluid that comes out of each of the cells 1, if coming from the cathodic chambers 30, comprises solution of NaOH and hydrogen gas and, if from the anodic chamber, comprises depleted brine and chlorine gas.

After separation of the gases, a part of the fluids leaves the equipment and a part is recircu- lated, being reintroduced into the cells after passage through the heat exchangers 300. This recirculation of liquids is produced by difference in density between columns inside the cells and by upward thrust that the bubbles of produced gas cause in the liquid inside the outlet tubes, so that each of them becomes a liquid recirculation pump.

These latter two phenomena are only possible in the equipment 100 of the present invention by virtue of the existence and combined operation of the heat exchangers 300 and of the fluid recirculation system, mentioned above.

The Teflon tubes or hoses 211 that link the outlet of the cells 1 to the pieces of tubing 203, 204, enable said separation of fluids and gases, which causes the phenomenon of pressuri- zation of the cell, mainly in the lower part of these tubes 211, exactly at the place where the fluids leave the cells.

The length and the areas of passage of the tubes 211 are determined so that the flow of gasses generated will perform the function of a pump moving a considerable volume of liquid from the cell to the respective upper tubing 203, 204. The liquid moved is replaced by feeding the cells from below through the lower tubing 201, 202, which has already been mentioned before.

These lower pieces of tubing provide about 90% of the liquid volume moved by dragging the gases mentioned in the preceding paragraph, completing the hydraulic circuit. The remaining 10% is fluid that is not the object of recirculation.

The lower tubing 201, 202 had their dimensions ideally calculated to enable correct feed- ing of the cells 1 and are made from materials that prevent galvanic corrosion (by difference in electric potential between the cells 1).

As mentioned before, the fourteen cells 1 are electrically connected in series, that is to say, the positive terminal of a fist cell is connected to the negative one of a second cell, the positive terminal of this second cell to the negative one of a third cell, and so on.

This type of connection in series (cathode-anode-cathode-anode, etc) configures the so- called bipolar electrode system and has the advantage of requiring little electric current for the production of soda, besides the distribution of the current being equal at all the cells 1.

When the equipment 100 is in operation, inside each electrolytic cell 1 the electrolysis reaction of the sodium chloride (NaCl) takes place for obtaining caustic soda.

The caustic soda is obtained by electrolysis from demineralized water and the electrolyte (brine = aqueous solution of sodium chloride). The chemical reaction that takes place inside the electrolytic cell is the following: 2NaCl + 2H₂O -» Cl₂ + H₂ + 2NaOH

Brine is introduced in the anodic compartment and, with application of electric current, the bindings of a part of the moleNaCl molecule is broken, giving rise to sodium ions (Na⁺) an Chlorine ions (Cl^"). The resulting brine, which is depleted, comes out of the electrolytic cell.

The chlorine ions (Cl^") are attracted to the anode due to their negative electric charge and lose one electron, given rise to the formation of chlorine gas Cl₂ (anodic reaction).

The sodium ions (Na⁺) are attracted to the cathode due to their positive electric charge, passing through the selective semi-permeable membrane that allows positive ions to pass, but prevents negative ions from passing.

In the cathodic compartment 30 said caustic soda is introduced in aqueous solution. The water in the cathode zone 3 forms the H⁺ and OH ions. The H⁺ ions, due to its electric charge, are attracted to the cathode 3, where they associate, forming hydrogen H2 molecules (cathodic reaction).

Therefore, inside the fourteen cells 1 of the equipment 100, the compounds obtained after the electrolysis are NaOH 32%, chlorine gas and hydrogen gas, besides the electrolyte, which is depleted.

A preferred example of an electrolyzer 100 for the production of caustic soda constituted with the teachings of the present invention (not limiting) has the following particular characteristics:

• it comprises 14 cells described before, electrically connected in series; • the feeding of the fluids of all the cells, and the removal of the electrolysis products is effected by means of a common tubing (the cells are connected in parallel with the tubing);

• a constant electric current of 2400 A is applied for a period of 12 months, obtaining a voltage per cell of about 3.8 V, which is ideal as a working charge, so that the equipment 100 works with approximately 53.2 V; • as an electrolysis product of the equipment one obtains 42.7 KTh of chlorine gas on the anodic surface, 1.26 K/h of hydrogen gas on the cathodic surface and 48.16 /h of caustic soda at 32% of high purity. The output of the equipment is of about 96%, similar to that obtained by bigger equipment of industrial level.

A preferred embodiment having been described, it should be understood that the scope of the present invention embraces other possible variations, being limited only by the contents of the accompanying claims, which include the possible equivalents.

Claims

1. An electrolytic cell (1), particularly designed for obtaining caustic soda (NaOH) and halogens by electrolysis, provide with at least one fist electric terminal (2) and at least one second electric terminal (3) positioned adjacent each other for carrying out the electrolysis, characterized in that the first electric terminal (2) is an anode, centrally positioned in the electrolytic cell, constituting an anodic chamber (20) and the second electric terminal (3) comprises two cathodes, each positioned laterally relative to the anode (2) and opposite each other and constituting a respective cathodic chamber (30), the anode (2) and each cathode (3) being separated by means of a semi-permeable membrane (4).

2. An electrolytic cell according to claim 1, characterized in that the anodic and cathodic chambers (20, 30) have internal space for the circulation of electrolyte fluid.

3. An electrolytic cell according to claim 2, characterized in that the electrolyte fluid enters the cell (1) through a first opening (5) provided centrally.

4. An electrolytic cell according to claim 2 or 3, characterized in that the fluids resulting from the electrolysis process leave the cell (1) through at least one second opening (6) provided centrally.

5. An electrolytic cell according to any one of the preceding claims, characterized in that the electric current for the electrolysis goes into cell (1) from the outer sides of the anode and comes out from the opposite sides of the cathodes after passing the electrolyte fluid and the semi-permeable membrane, the anode and cathodes having copper connections (2', 3') suitable for the purpose.

6. An electrolytic cell according to any one of the preceding claims, characterized in that the cathodes (3) are formed from 316L stainless steel, while the anode (2) is formed from activated titanium.

7. Electrolyzing equipment (100), particularly for the production of caustic soda (NaOH) and halogens by electrolysis, provided with at least one body portion (200), which defines an internal cavity for positioning and operating at least one electrolytic cell (1), the equipment being characterized in that:

(i) the electrolytic cell (1) is provided with at least one first electric terminal in the form of an anode (2) positioned centrally, constituting an anodic chamber (20) and at least one second electric terminal in the form of two cathodes (3), each positioned laterally relative to the anode (2) and opposite each other and constituting a respective cathodic chamber (30), the anode (2) and each cathode (3) being separated by means of a semi-permeable membrane (4); (ii) it comprises at least one integrated recirculation system for enabling operation of the electrolytic cell (1), which:

(11.1) operates by difference in temperature and has at least two heat exchangers (300) incorporated therein;

(11.2) operates by difference in density between columns of fluid; enabling pre-heating and maintenance of the operation temperature of the equipment and the uniformity in the concentration of the electrolyte fluid in the anodic and cathodic chambers (20, 30) of the cell (1).

8. Electrolyzing equipment according to claim 7, characterized in that the integrated recirculation system comprises at least two heat exchangers (300) associated to the body portion (200) and means for injecting cold or heated water into these heat exchangers (300).