Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B15/00—Operating or servicing cells
  • C25B15/08—Supplying or removing reactants or electrolytes; Regeneration of electrolytes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/34—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis
  • C25B1/46—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis in diaphragm cells

Definitions

• 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.
• 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.
• 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).
• ultrapur e is used to describe a brine of approximately the following composition:
• 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).
• caustic potash potassium hydroxide or KOH
• 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.
• 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.
• 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).
• JECFA Joint FAO/WHO Expert Committee on Food Additives
• ECHA European Chemicals Agency
• 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.
• 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.
• this technology has limitations in that it allows the content of chlorates in potash/soda to be kept below 50 ppm.
• 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:
• 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.
• 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.
• 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.
• sodium sulphite is used as a reducing agent for chlorates in an acidic environment.
• the concentration of chlorates is reduced to the order of 100 mg/L.
• the aim of this invention is to provide a satisfactory response to the technical problem described above.
• 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.
• 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;
• FIG. 1 shows a schematic diagram of the process in accordance with the prior art
• FIG. 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
• 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):
• 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.
• FIG. 2A 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.
• 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.
• 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 ;
• 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.
• a half- saturated brine tank not shown in Fig. 2
• 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.
• 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).
• Two polypropylene piston pumps with a nominal flow rate of 5 m 3 /h and a pressure of 10 bar are used.
• the running capacity is adjustable by means of an inverter located on the motors.
• the pump's capacity is regulated by an inverter located on the motor.
• a palladium titanium plate heat exchanger was advantageously selected, which proved better than a solely titanium plate heat exchanger.
• the preferred validated material was enamelled steel.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the residual quantity of chlorates is less than 30 ppm, substantially zero ppm, or completely zero ppm.

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• 2019
  • 2019-06-06 EP EP19736507.5A patent/EP3802919A1/de active Pending
  • 2019-06-06 WO PCT/IB2019/054702 patent/WO2019234665A1/en unknown

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