PRODUCT SUBSTANTIALLY FREE FROM CHLORATE IONS

DESCRIPTION

Technical Field of the Invention

In the context of the technology for the production of caustic potash (in solution or as a solid phase) from potassium chloride by an electrolytic process with membrane cells, the present invention is aimed at a process of dechloratation of KC1 brine, in particular half- saturated KC1 brine, which enables a KOH-based end product to be obtained on an industrial scale, such as, for example, 30% w/w or 50% w/w KOH in aqueous solution (potash), or solid KOH, containing the smallest possible quantity of chlorate ions, in particular less than 30 ppm, substantially devoid of chlorates, or completely devoid of chlorates.

Scope of the Invention - Brief Description of the Prior Art

At the present time, potassium chloride is industrially converted by a process of electrolysis with electrolytic membrane cells, in particular semi-permeable membrane cells, primarily into caustic potash in aqueous solution, hydrogen and gaseous chlorine, from which are produced the related potassium and chlorinated derivatives.

Until the end of the twentieth century, the technology prevalently adopted, particularly in Europe, for the production of potash, hydrogen and chlorine was mercury amalgam electrolysis. This technology was gradually abandoned and then definitively banned by the European Community from December 11 2017, to be replaced first by the diaphragm technique, and then by the much more effective membrane technology, specifically semi-permeable membranes, which has proved to be the least environmentally impacting and the most energy efficient (as evidenced in the sector specifications of Best Available Technology [B.A.T.], Reference Document for the Production of Chloro-Alkali, European Commission 2014). Potassium chloride, or KC1, which is the starting raw material of the above process, is commercially acquired in solid form with an average purity of 99.5%, from basins in Germany, Russia, Belarus, Canada, Jordan, and is dissolved in water prior to the use until it reaches a saturation concentration of 300 g/L (hereinafter this solution is designated, for convenience, saturated brine ) and is then appropriately purified until it reaches the degree of purity that makes it suitable to be electrolyzed in accordance with the semi-permeable membrane cell technique (hereinafter this solution is designated, for convenience, ultrapur e saturated brine).

The term " ultrapur e" is used to describe a brine of approximately the following composition:

Ca + Mg < 20 ppb

Sr < 50 ppb

Ba < 0.5 ppm

Si0 ₂ < 5 ppm

Al < 0.1 ppm

Fe < 0.2 ppm

I < 0.2 ppm Ni < 10 ppb

Mn < 0.01 ppm

Cr < 1 ppm

Hg absent

TOC < 10 ppm

S0 ₄ < 8 g/l

KCIO3 < 15 g/l

Solid Susp. < 1

Active chlorine Absent

In the electrolytic cells, therefore, the ultrapure saturated brine (also called "feed brine ", in Fig. 1 attached) enters from the anodic side: the chloride ion (C1-) is oxidized by electrolysis at the anode to chlorine gas (Cl ₂ ) while the H+ ion is reduced at the cathode to hydrogen gas (FF).

The OH ion remains in solution in the cathodic compartment (the membrane being impermeable to it) and reacts with the K ⁺ ion, which instead crosses the membrane (permeable to it) passing from the anodic compartment to the cathodic compartment, to form caustic potash (potassium hydroxide or KOH), which exits from the cathodic compartment in the form of an aqueous solution at a concentration of 30% by weight (w/w) (as illustrated in Fig. 1 attached).

Once electrolyzed, the ultrapure saturated brine exits from the anodic compartment at a concentration of about 180 g/L (hereafter, this brine is referred to as half-saturated brine , or " depleted brine see Fig. 1 attached), and is directed to the saturation tank (shown in Fig. 2), to be re-saturated with new solid KC1 salt until it again reaches the concentration of 300 g/L, after which it is again ultra-purified, and finally returns to the electrolytic cell (as ' feed brine") to produce again chlorine, hydrogen and caustic potash. This production cycle is virtuously defined as a "closed circuit/cycle", since there is no discharge of exhausted effluent out to the exterior.

In the context of the technology described above, the purification of the potassium chloride brine to give the ultrapure saturated brine, or "feed brine ", plays a fundamental role, because it affects two fundamental aspects; these are i) the energy efficiency of the electrolytic process and ii) the containment of the unwanted formation of toxic oxygenated compounds of chlorine, of which the most important is the chlorate ion, CIO3 .

The chlorate ion is an ion species that is formed in the anodic compartment, during the electrolysis stage of the ultrapure saturated brine, as a result of a parasitic reaction (intrinsic to the process) between two chlorite ions (ClO ), present due to the massive concentration of chlorine at elevated temperatures. The chlorate ion is significantly harmful in many respects. Indeed, being a highly stable compound, the chlorate ion tends on the one hand to accumulate in the anodic side of the electrolytic cell, thus concentrating within the half- saturated brine (" depleted brine") and becoming more and more present in the closed cycle of the brine. This hinders the purification of the KC1 brine (it limits the solubility of KC1, is corrosive to the materials used and, above certain concentrations, reduces the efficiency of the current). On the other hand, it crosses the electrolytic membrane by diffusivity (in an amount proportional to the concentration in which it is present in the brine within the anodic compartment), passing from the anodic compartment to the cathodic compartment and thus significantly polluting the finished product (that is, the caustic potash and consequently also its subsequent derivatives), which, as already mentioned, leaves the cathodic compartment of the electrolytic cell in the form of an aqueous solution of KOH at a concentration of 30% by weight (w/w) and can subsequently be concentrated and sold as a 50% w/w aqueous solution or under a solid form, for example, between 90-92% w/w or more, as well as its potash derivatives (potassium carbonate, in solution and/or as the solid).

The chlorate ion is also a redoubtable pollutant for the food chain (for example, it is a suspected triggering agent of diseases of the thyroid and the nervous and circulatory systems) and for everything associated therewith, for example, also as an additive in food, agri-food, fertilizer (such as KOH and its potash derivatives).

For this reason, the chlorate ion has recently been increasingly subjected to rigorous and stringent regulation. For example, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) and the European Chemicals Agency (ECHA) have assessed the health risk from eating food contaminated with chlorates, setting a value of 0.06 mg/kg body weight as an acceptable daily intake (ADI).

As a result, Regulation 2005/396/EC has set the maximum acceptable level of chlorate in food at 0.01 mg/kg. However, to date, there are no official regulations indicating the maximum permissible amount of chlorate ion in KOH (both in solution and under solid form) and in its potash derivatives.

Today's membrane electrolysis plants, which meet the requirements of the Best Available Technology (B.A.T.; Reference Document for the Production of Chloro-Alkali , European Commission 2014) guarantee, by technical specification, a maximum concentration of CIO3 ions (in the form of KCIO3) in aqueous KOH at 30% w/w on exiting from the electrolytic cells equal to 30 ppm. This is, to date, the state of the art of the world's best technology available on the semi-permeable membrane electrolysis market that produces chloro- hydrogen potash. Any manufacturer of KOH using membrane technology, with the production plant started, can only guarantee this specification of purity to its customers, and nothing more.

Unfortunately, in light of the above, the above specification limit, as guaranteed by suppliers in accordance with Best Available Technology (B.A.T.), is not yet satisfactory, neither for those who wish to supply a product of a quality suitable for entering the food chain, nor, at the same time, for those end users who are looking for a potash product (for example, based on KOH) with the lowest possible amount of chlorate ion impurities.

The following relevant prior-art documents illustrate some of the procedures that can be used, and are used in practice, to reduce the presence of undesirable ClOyions in KOH in the industrial production of caustic potash (in solution or as a solid phase) from potassium chloride, by means of an electrolytic membrane-cell process.

US 4481088A describes the dechloratation technology that is used in almost all of the chloro-alkali electrolysis plants (this is what membrane plant manufacturers recommend). The dechloratation process is applied to the saturated brine because it is said that this will positively affect the chlorate reduction reaction, decreasing the excess of HC1 required for the reaction and increasing its rate. Furthermore, it is stated that operating on the brine in a different way, it would be necessary to provide a dechlorination system dedicated to the outlet flow of the dechloratation. However, the use of saturated brine in the treatment of chlorates can lead to over-saturation of the brine and therefore to precipitation of the salt with the resultant plant problems. Furthermore, this technology has limitations in that it allows the content of chlorates in potash/soda to be kept below 50 ppm. In addition, the plant operator must decide how much brine to treat because the greater the amount treated, the higher the costs (HC1 that must necessarily be neutralized with hydroxides of K and Na, which have a certain cost). It follows that for chloro-potash falling below 10,000 ppm in brine (corresponding to 30 ppm in potash) becomes uneconomical.

US 4269773 describes a dechloratation process that is applied to the semi- saturated brine leaving the anodic compartment of an electrolytic cell, diverting part of the flow normally directed to dechlorination towards the dechloratation reactor. The dechloratation reactor is equipped with a source of UV rays to break down CIO2 into chlorine and oxygen. This gas is formed under the conditions described in this patent in not insignificant amounts, according to the following reaction:

HC10 ₃ + HCl ® C10 ₂ + 0,5 Cl ₂ + H ₂ 0

The chlorine produced in the reactor is either brought down with a dedicated NaOH solution or it is conveyed to the chlorine produced by the electrolytic cell. The solution exiting the dechloratation is divided into two streams (without specifying the percentage amount thereof, which is not negligible), one directed at exploiting the residual acidity of the solution to adjust the pH of the anolite before placing it in the cell and one directed to a crystallizer. The crystallized salt is used to re-saturate the brine. However, the process of crystallization of the salt exiting from the dechloratation section is an energetic process and involves a purge of the solution, as can be discerned from the figure in the patent. In addition, US 4269773 also does not describe a final product containing less than 30 ppm of chlorate ions.

US 4609472 describes the treatment of impure brine and uses, in addition to HC1, another reagent, N2H ₄ ·HO, which brings with it, in addition to an additional cost in the process as a whole, further safety issues, in particular - as it is toxic - potentially carcinogenic, and also toxic to aquatic organisms with long-lasting effects. Consequently, US 4609472 does not present as a preferred alternative for industrial production. Furthermore, US 4609472 also does not describe an end product containing less than 30 ppm of chlorate ions.

US 2010/219372 Al describes a treatment to remove organic and inorganic impurities at acceptable values to use in electrolysis a brine that could be derived for example from the GTE (glycerol to epichlorohydrin) process. The patent describes the purification of a brine containing organic impurities that can be removed by electrochemical oxidation. This leads to the formation of chlorates and/or hypochlorites as unwanted products. In this case sodium sulphite is used as a reducing agent for chlorates in an acidic environment. In examples 1 and 2 given in this patent, the concentration of chlorates is reduced to the order of 100 mg/L.

In this case, the use of sodium sulphite as a reducing agent produces sulphates in solution. These sulphates can accumulate in the cycle, generating new additional costs of purification. In addition, US 2010/219372 Al does not describe a final product containing less than 30 ppm of chlorate ions. Technical Problem

There remains within the technical field the need for a KOH-based product such as, for example, KOH in aqueous solution (potash) at a concentration of 30% w/w or 50% w/w, or as a solid, for example with a purity of at least 90% or higher (for example, from 90 to 92 % or higher), or a potassium derivative thereof (such as, for example, potassium carbonate, in solution or as a solid) containing the smallest possible quantity of chlorate ions (for example, in the form of Cl0 ₃ , or KCIO3), in any case less than 30 ppm; preferably, it is substantially chlorate-free (i.e., with the chlorate ion concentration substantially = 0 ppm); even more preferably it is completely chlorate-free (i.e., with the chlorate ion concentration = 0 ppm).

The aim of this invention is to provide a satisfactory response to the technical problem described above.

Summary of the invention

The applicant has now found that a suitable process to degrade the KCIO3 (hereinafter, the "dechloratation" process) present in the KC1 brine, in particular in the half- saturated KC1 brine, performed using a suitable reactor/plant dedicated to this process, is capable of giving an adequate response to the technical problem generated by the need described above.

Therefore, an object of the present invention is a process of dechloratation of KC1 brine, as stated in the attached independent claim.

Another subject of the present invention is a dedicated reactor to carry out the above-mentioned dechloratation of KC1 brine, as stated in the attached independent claim. A further subject of the present invention is a KOH-based product that is substantially or completely devoid of chlorate ions, obtained using the above process and reactor, as stated in the attached independent claim.

These and other aims are therefore achieved with the present process for the dechloratation of the half- saturated KC1 brine of/from the anodic compartment of an electrolytic cell, during the process of producing potash-chloro-hydrogen from KC1 by means of membrane technology, to give a finished product based on KOH, in aqueous solution or in solid form, substantially or completely devoid of impurities of chlorate ions, said process comprising:

- drawing out, downstream the under-vacuum dechlorination section of the half- saturated brine, an amount of dechlorinated half- saturated brine, preferably having pH from 2 to 3; said dechlorinated half-saturated brine being in an amount ranging from 1 to 20 m ³ /h;

- superheating said dechlorinated half- saturated brine through a heat exchanger utilizing vapour at a pre-determined pressure as the heating fluid; said superheating temperature being from 40 °C to 200 °C and said vapour pressure being from 8 to 12 bar;

- introducing into said superheated brine, downstream the heat exchanger, HC1 at 32-37 % in excess, with respect to the dechloratation reaction (1) stoichiometry; said excess HC1 being in a two-fold excess or more, with respect to the said dechloratation reaction (1) stoichiometry;

- conveying into a dedicated dechloratation reactor the superheated an acidified brine and letting the dechloratation reaction occur at a temperature > 80 °C, from 80 °C to 220 °C;

in which the residual amount of chlorate ion impurities is at least < 30 ppm. Further objects of the present invention are described in the dependent claims.

Brief Description of the Drawings

Further characteristics and advantages of the present process and relating to the plant, according to the invention, will become more apparent from the following description of a few embodiments thereof, provided by way of non-limiting examples, with reference to the accompanying drawings, wherein:

- Fig. 1 shows a schematic diagram of the process in accordance with the prior art;

- Figs. 2A and 2B show the process according to the invention as a flowchart;

- Fig. 3 is a graph that shows a trend of the rates of reaction as the temperature changes;

Detailed description of the invention

The present invention is aimed at a process for the dechloratation of KC1 brine, in particular of half-saturated KC1 brine at a concentration of 180 g/l of/from the anodic compartment of an electrolytic cell, in the process of producing potash- chloro-hydrogen from KC1 by means of membrane technology, to give a finished product based on KOH substantially, or practically, or completely devoid of impurities represented by chlorate ions (in the form of free ions in solution or of KCIO3), wherein the chlorate ions are present in a quantity of 0 to 5 ppm or less.

The process of the present invention is based on the following chemical reaction of decomposition of KCIO3 (1):

KCIO3+ 6 HC1 + Heat (150 °C) = 3 Cl ₂ + KC1 + 3H ₂ 0 (1) According to this reaction, the chlorate ion (present with its counter-ion K ⁺ , in solution) breaks down as a function of the temperature and concentration of HC1.

As these parameters (HC1 concentration and reaction temperature) increase, the rate of the decomposition reaction increases.

The process of the present invention is based on the almost complete or complete destruction, according to the reaction (1), of the chlorate ion present in the half- saturated KC1 brine (or " depleted brine") coming from the step of "dechlorination under vacuum" to which the half- saturated KC1 brine coming out of the anodic section of the electrolytic cell is subjected.

In Fig. 2A attached, for the sake of completeness of information there is a schematically representative block diagram of the KC1 brine purification plant, wherein, connected by orange arrows, the section of the vacuum dechlorination phase of the half-saturated KC1 brine leaving the anodic section of the electrolytic cell and the section of the dechloratation step (carried out using the dedicated dechloratation reactor, object of the present invention; see in this regard Fig. 2B) of said half- saturated dechlorinated brine, subject of the present invention.

The following description of the dechloratation process of the invention is given only by way of non-limiting example, with reference to a dechloratation reactor with a geometric volume of 2 m ³ . The person skilled in the art within this sector will obviously be able to make, in the light of his technical knowledge, the appropriate changes to the system, if production is carried out on a larger scale using a similar reactor suitably sized on an industrial scale.

In accordance with the dechloratation process of the present invention: - downstream the vacuum dechlorination section, for example, a suitable quantity of half-saturated dechlorinated brine (pH 2 to 3; preferably 2.3 to 2.7; more preferably about 2.5; even more preferably = 2.5) is drawn off by means of a piston pump or similar, preferably in a quantity varying from 1 to 20 m ³ /h or more; more preferably 1 to 15 m ³ /h; even more preferably 1 to 10 m ³ /h;

- said dechlorinated brine is superheated (preferably from 40 °C to 200 °C; more preferably, from 50 °C to 190 °C; even more preferably, from 50 °C to 180 °C), for example, by means of a heat exchanger (not illustrated in Fig. 2) which uses as the heating fluid pressurized steam at a pre-determined pressure; this pressure being comprised within the range 8 to 12 bar; preferably, at about 10 bar; more preferably, at 10 bar; the brine temperature is managed by an automatic control loop ;

- downstream the exchanger, an excess of 32-37% hydrochloric acid with respect to the stoichiometry of reaction (1) is introduced into the circuit; preferably, in an excess of 2 times or more; more preferably, in an excess of 3 times or more; even more preferably, in an excess of 4 times, optionally even more, via an adapted dosing pump having, for example, a maximum flow rate of 600 1/h, or more (not shown in Fig. 2);

- the superheated and acidified brine is fed into the dedicated dechloratation reactor (having, for example, a geometric volume of 2 m ³ ) and the dechloratation reaction (1) is carried out at an operating temperature > 80°C; preferably comprised within the range 80°C to 220°C; preferably 90°C to 200°C; more preferably l00°C to l90°C; even more preferably H0°C to l80°C, depending on the excess of hydrochloric acid to be used.

A calibrated disc/valve (not shown in Fig. 2) is applied to the top of the reactor to allow the release of the gases (chlorine and water vapour) resulting from the reaction of KCIO3 decomposition (Reaction 1), which are continuously directed to the chlorine washing column. The level of the dechloratation reactor is suitably controlled by means of a control loop.

The dechloratated half-saturated brine leaving the dechloratation reactor flows, as treated and almost devoid of residual chlorate impurities, into an expansion vessel (not shown in Fig. 2), and from there, returns by gravity to a half- saturated brine tank (not shown in Fig. 2), which is located upstream the dechlorination section under vacuum. In summary, by appropriately modulating the temperature (and, consequently, the operating pressure) and the degree of free acidity, it is possible to obtain the desired yield from Reaction (1). Once returned into the dechlorination section under vacuum, the half- saturated brine, substantially or completely dechloratated, is subjected to the conventional steps of saturation, decantation, filtration and purification to give the brine of ultrapure saturated KC1, substantially or completely dechloratated, which is fed back to the anode sector of the electrolytic cell to give Cl ₂ , H ₂ and KOH in 30% w/w solution, substantially or completely devoid of impurities of chlorate ions or of KCIO3.

Purely by way of an illustrative, albeit non-limiting example of the possible embodiments of the present invention, a preferred mode of operation of the dechloratation system/plant of the invention is given below in greater detail.

Experimental example: Realization of the dechloratation reaction of the invention of the system and of the reactor dedicated thereto

Conditions of the reaction

The dechloratation reaction is Reaction (1) described above. The operating temperature of the reaction in the dedicated reactor is comprised within the range 1 lO°C to l80°C.

The dosage of the solution of 32-37% w/w HC1 is in excess by a factor of about 4 with respect to the stoichiometry of Reaction (1).

The higher the operating temperature, the lower the necessary excess of hydrochloric acid, as results by the typical operating values that have been experimentally extrapolated to obtain a yield from Reaction (1) higher than 95%, preferably higher than 98%, on a treated flow rate of 7 tons/h of dechlorinated half-saturated brine, containing an average value of Cl0 ₃ equal to 1.3 g/L.

T=l20 °C; quantity of brine acid at dechloratation: HCl=2.3% w/w

T=l30 °C; quantity of brine acid at dechloratation: HCl=l.7 % w/w

T=l45 °C; quantity of brine acid at dechloratation: HCl=l.5 % w/w

Pumping o f the half-saturated brine

Two polypropylene piston pumps with a nominal flow rate of 5 m ³ /h and a pressure of 10 bar are used.

Polypropylene is well resistant to acidity corresponding to a pH of 2 to 3, in particular 2.3 to 2.7; especially about 2.5, optimally at pH=2.5, and at a temperature of 50 °C. The running capacity is adjustable by means of an inverter located on the motors.

Dosage of 32-37% w/w hydrochloric acid solution

It is carried out by means of a Teflon membrane dosing pump: the pump's capacity is regulated by an inverter located on the motor.

Superheating of the half-saturated brine from 50°C to 180°C

A palladium titanium plate heat exchanger was advantageously selected, which proved better than a solely titanium plate heat exchanger.

Materials for piping and reactor under conditions of high temperature and acidity

The preferred validated material was enamelled steel.

The construction difficulties are due to the fragility of enamelled steel. It was therefore necessary to focus most closely on the aspects of thermal expansion and, therefore, the brackets; an important aspect concerned the choice of gaskets: among these, Teflon gaskets with a klingerit core were tested and adopted. Other gaskets of interest were Teflon gaskets with graphite core; fiberglass-covered PVDF (for temperatures up to l40°C and p=4 bar); Teflon- coated steel.

Continuous extraction o f exhaust gas

The continuous extraction of the gaseous reaction products (or off-gas ), chlorine (Cl ₂ ) and water vapour (H ₂ 0), was carried out by mounting on the upper end of the reactor a Teflon calibrated disk: the diameter of the orifice was determined empirically by testing disks with various holes sizes, until one is found that best suits the average conditions of the operating potential, thus allowing the off-gas to be extracted safely without altering the pressure inside the reactor.

For this purpose:

- the automatic level control valve is made of Teflon-coated steel;

- the well for housing the thermistor for temperature measurement is made of tantalum;

- the pressure gauges for measuring reactor pressure and reactor level are made with a tantalum sensor;

- the pressure relief valve in the reactor is made of Teflon-coated steel. Electronic process control

The system is controlled from the control room by means of an electronic processor, from which it is possible to carry out the operations of stopping, starting and adjusting the conduction parameters.

When there is an interruption in the electricity supply, there is also a lack of steam to the plant: in the event that even the pump supplying the half- saturated brine stops, one would be faced with a static situation, in which phenomena of diffusion of strong acidity could occur; this could also affect the exchanger (titanium palladium), impairing its integrity.

To avoid this phenomenon, an automatic system has been set up that intervenes by immediately starting the 2 pumps powered by electricity from the generator set. In this way, the system is secured.

The reactor according to the present invention is able to destroy the whole quantity of KCIO3 that is generated within the anodic compartment during the electrolysis of the saturated ultrapure KC1 brine. As already described above, the lower the chlorate content in the ultrapure saturated KC1 brine fed to the electrolytic cell, the lower the amount of chlorates that migrates due to diffusivity into the cathodic compartment, and will therefore contaminate the 30% w/w KOH solution at the outlet therefrom and the subsequent conversion products thereof such as, for example, the 50% KOH solution and/or the final solid KOH, and any other conversion products thereof.

The KOH obtained by the process of the present invention has an impurity content of chlorate ions (considered as such or as KCIO3) on average comprised within the range 0 to 5 ppm; preferably 0 to 3 ppm; more preferably 0 to 2 ppm or less. Innovativeness of the Process of the Invention

Using the Arrhenius equation

v = A exp(- AEa/RT),

wherein v is the rate of the reaction, R is the constant of perfect gases, T is temperature (in degrees Kelvin), AEa is the activation energy of the reaction and A is the pre-exponential factor (determined experimentally), it was possible to evaluate the kinetics of the reaction of decomposition of KCIO3 (Reaction 1) under variations in temperature. A simulation has therefore been carried out in this sense, where: A = 2,403 · 10 ² and AEa = 81,84 kJ/mol.

From the graph shown in Fig. 3 attached, it can be seen that, working at 150 °C (valued temperature based on the measurement of the internal pressure of a suitable reactor equal to about 4.5 bar), a rate about 11 times greater than that relating to a working temperature of 110 °C is obtained. An even better condition is a temperature of 160 °C, which exhibits a rate about 19 times that achievable at 110 °C, while the pressure should rise to about 6.14 bar, applying the Antoine equation for water (¾ ftp .7/w w . water vaporpress ure. com/) . The increase in temperature is therefore accompanied by a significant increase in rate of reaction, compared to a relatively small increase in pressure, as shown in

Table 1 below.

Table 1

100 1,02 0,50

110 1,44 1,0

120 1.98 1.9

130 2,69 3,6

140 3,59 6,5

150 4.73 11.3

160 6,14 19.4

170 7.S7 32.4

180 9.98 52.9 The dechloratation plant (including the dedicated reactor of the invention, as described above) was preferably inserted at a point of the KC1 brine purification cycle at which the residual acidity of the aliquot part of brine treated by dechloratation (it will be recalled that the dechloratation section operates with a strong excess of acidity) is reused in the conventional dechlorination section under vacuum. This section treats the half- saturated brine leaving the electrolytic cells, exploiting the high vacuum (pressure of 0.8 bar lower than atmospheric pressure), a high temperature, up to 60 °C - 80 °C, and acidity (preferably a pH=2.5) to eliminate the chlorine gas contained therein.

Before the invention of the dechloratation section that is the object of the present invention, it was necessary to add a significant quantity of auto-produced synthetic HC1, to reach pH=2.5 in the dechlorination section.

Therefore, the introduction of the inventive dechloratation section resulted doubly advantageous as it also serves to reduce internal auto-consumptions of chemical products (HC1 in this case) that are strategic for the industrial productions of the plant.

Advantages of the Process of the Invention

As already demonstrated above, the process of the present invention has shown unexpected and not negligible advantages over the known art.

In particular, in a comparison with US 4481088 A, following the process that is the object of the present invention, there is no requirement for longer dwell times in the dechloratation reactor, reduced quantities of HC1 comparable to or lower than those proposed by US 4481088A are used, but with unexpectedly better dechloratation yields. In particular, the residual amount of chlorates is less than 30 ppm, substantially zero ppm, or completely zero ppm.

As compared with US 4269773, following the process that is the object of the present invention the formation of measurable quantities of CIO2 is not observed. It is therefore not necessary to use other energy to power a source of UV light to decompose this gas. In addition, the absence of CIO2 facilitates management of the gases at the reactor outlet, it being possible to direct them into the chlorine stream produced by the electrolytic cells. The portion of treated half- saturated brine containing chlorine produced by dechloratation, can be safely sent back to dechlorination without requiring a dedicated section. The efficiency of the reaction carried out in accordance with the teaching of the present invention means that it is not necessary to further purify the brine to obtain the results of chlorate depletion to well below 30 ppm, as claimed in claim 1 attached.

As compared with US 4609472, the process which is the object of the present invention is distinguished by the lack of toxicity, potential or actual, caused by the use of N ₂ H ₄ TTCI, the lack of the related increase in production costs, to obtain the abatement of chlorates well below 30 ppm, as claimed in claim 1 attached.

With respect to US 2010/219372 Al, the process which is the object of the present invention does not use sodium sulphite to eliminate chlorates and, therefore, does not produce sulphates in solution with the relative increase in production costs generated by the accumulation thereof. In addition, the residual quantity of chlorates is less than 30 ppm, substantially zero ppm, or completely zero ppm. Industrial Applicability

In the context of the industrial production of caustic potash (in solution or as a solid phase), hydrogen and gaseous chlorine from potassium chloride by an electrolytic process with membrane cells, the process of dechloratation of half- saturated KC1 brine of the present invention has enabled a KOH-based end product to be obtained on an industrial scale, such as, for example, 30% w/w or 50% w/w KOH in aqueous solution (potash), or solid KOH, or a potassium derivative obtained therefrom, containing the smallest possible quantity of chlorate ions, in all cases less than 30 ppm, substantially devoid of chlorates, (i.e. with the chlorate ion concentration substantially = 0 ppm); or completely devoid of chlorates (i.e. with the chlorate ion concentration = 0 ppm).