Process for Electrolytic Production of Alkali Metal Chlorate and Auxiliary Chemicals

Home , Chlorate

Europaisches Patentamt 19 European Patent Office

Office europeen des brevets © Publication number: 0 498 484 B1

12 EUROPEAN PATENT SPECIFICATION

@ Date of publication of patent specification (6) Int. CI.5: C25B 1/26 09.06.93 Bulletin 93/23

(2j) Application number: 92200201.9

(22) Date of filing : 24.01.92

@) Process for electrolytic production of alkali metal chlorate and auxiliary chemicals.

(30) Priority: 05.02.91 SE 9100365 @ Proprietor : EKA NOBEL AB S-445 01 Bohus (SE)

(43) Date of publication of application 12.08.92 Bulletin 92/33 (72) Inventor : Wanngard, Johan Linjevagen 25 S-854 68 Sundsvall (SE) (45) Publication of the grant of the patent : Inventor : Carlsson, Arne 09.06.93 Bulletin 93/23 697 Matson Road Columbus, MS 39701 (US) Inventor : Backstrom, Jan Erik Tatortsvagen 1 @ Designated Contracting States : S-445 45 Nodinge (SE) DE ES FR PT SE

(74) Representative : Schold, Zaid (56) References cited : Nobel Industries AB Patent Department Box FR-A- 2 607 723 11554 S-100 61 Stockholm (SE)

CO oo

Note : Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been LU filed until the opposition fee has been paid (Art. 99(1) European patent convention).

Jouve, 18, rue Saint-Denis, 75001 PARIS 1 EP 0 498 484 B1 2

Description most of the process steps correspond to the equivalent process steps in the production of sodium chlor- The invention relates to a process to limit the con- ate. Thus, usually sodium chloride electrolyte is elec- tent of impurities in the production of alkali metal trolyzed to sodium chlorate and sodium hydroxide. chlorate, at which the production of chlorate is inte- 5 Potassium chlorate is formed by addition of a purified grated with the production of chlorine and alkali metal solution of potassium chloride to the sodium chlorate hydroxide, which auxiliary chemicals are used in the solution formed. Subsequently, the crystallization of chlorate process. The alkali metal chlorate is pro- potassium chlorate takes place, usually by cooling duced by electrolysis of a purified electrolyte contain- and evaporation. ing alkali metal chloride, alkalization of the chlorate 10 In the cyclic chlorate process the pH is regulated electrolyte obtained and precipitation of the chlorate in several positions within the range 5.5 - 12, to opti- formed by evaporation of the chlorate electrolyte. mize the process conditions in each unit operation. Suitably, the very pure water separated in the crys- Thus, a weakly acidic or neutral pH is used in the elec- tallizer is used in a membrane or diaphragm cell in the trolyser and reaction vessels to promote the forma- production of chlorine and alkali metal hydroxide, 15 tion of hypochlorite, while the pH in the crystallizer is partly as a raw material directly in the cathode com- alkaline to avoid the reaction of hypochlorite to chlor- partment and partly to prepare the chloride electro- ine instead of to chlorate and also to reduce the risk lyte, together with alkali metal chloride. Where appro- of corrosion. priate, the pure water is also used in the production Normally hydrogen chloride is used to lower the of hydrochloric acid by absorption of hydrogen chlor- 20 pH, but also chlorine is used completely or partly. ide after a hydrogen chloride burner. The alkali metal Normally, alkali metal hydroxide is used to make the hydroxide produced is used in the alkalization of solutions alkaline. Hydrogen chloride and alkali metal chlorate electrolyte, in the gas scrubbers and in the hydroxide are added as aqueous solutions. Commer- purification of the technical salt fed. In acidification, cially available technical solutions of hydrochloric either pure chlorine or hydrogen chloride can be used, 25 acid and alkali metal hydroxide contain impurities, at which hydrogen chloride is produced from chlorine which even at low contents are unfavourable for the and hydrogen generated in the process. The amount chlorate electrolysis. of alkali metal hydroxide produced in the process, is A chloride electrolyte to be electrolysed in a chlor- preferably equivalent to the main consumption in the ate cell, must not contain high contents of impurities. production of chlorate. 30 Thus, Ca^, Mg2+ and SO42" cause depositions on the cathodes and thereby a higher operating voltage and Background energy costs, while heavy metals decompose the hypochlorite formed to chloride and oxygen, instead of, Alkali metal chlorate, and particularly sodium as desired, to chlorate. To avoid these drawbacks, the chlorate, is an important chemical in the cellulose in- 35 chloride electrolyte is normally purified, which most dustry, where it is used as a raw material in the pro- simply takes place already in the preparation of brine duction of chlorine dioxide, which is an important by dissolution of the technical salt. In this part of the bleaching chemical for cellulose fibres. Alkali metal process, the flow is small and chlorine compounds chlorate is produced by electrolysis of an electrolyte such as molecular chlorine and chlorate have not yet containing alkali metal chloride according to the over- 40 been formed. The impurities may also be removed lat- all formula: er in the process before the chloride electrolyte is in- MeCI + 3H20-> MeCI03 + 3 H2 troduced into the electrolysers. The purification can (Me = alkali metal) be brought about by the addition of chemicals con- The process is cyclic, where in a first step the chloride taining CO32", OH" and Ba2* for the precipitation of electrolyte is brought to an electrolyser for the forma- 45 e.g. calcium carbonate, magnesium hydroxide and tion of hypochlorite, whereupon the solution is barium sulphate and by letting the brine or chloride brought further to reaction vessels for further reac- electrolyte pass ion-exchange resins where addition- tion to chlorate. Subsequently, chlorate formed is al Ca^, Mg^ and also Ba2* are removed. Suitably, al- separated by crystallization. The crystallization of kali metal hydroxide is used in the purification of brine chlorate can be brought about by evaporation or cool- 50 and the regeneration of ion-exchange resin. ing. Evaporation means that the water is evaporated Normally, a chloride electrolyte to be electrolysed and condensed in a separate step, either indirectly in in a chlor-alkali process, must also be purified from a heat exchanger or, more frequently, directly in the impurities. This is especially valid for membrane cells, cooling water. Cooling crystallization means that the where magnesium and calcium hydroxide can be pre- temperature is lowered to such an extent, that the ss cipitated in the membrane and cause increased oper- chlorate electrolyte becomes saturated with chlorate ating voltage and reduced current yield, if not sub- whereby crystals precipitate. stantial purification measures are brought about. In In the production of potassium chlorate, usually this connection, the chloride electrolyte or brine are 2 3 EP 0 498 484 B1 4 treated in a similar way as the solution intended for material or discharge of unwanted substances from the chlorate electrolysis, i.e. by precipitation with the process after that they have been allowed to con- chemicals and separation followed by ion exchange. taminate the chloride electrolyte and electrolyte of va- Suitably, alkali metal hydroxide is used also here. In rious concentrations. The discharge occurs either by this case, the chlorine content in the recirculating 5 one or more purification steps in the process or by the chloride electrolyte must be reduced down to the ppm impurities accompanying the products. To avoid ac- level, since presently available ion-exchange resins cumulation of impurities in the chlorate electrolysis in the final purification step are not resistent to free and with that an accompanying requirement for puri- chlorine. In this connection, vacuum is used in one or fication measures, is what this invention aims at solv- more steps, adsorption on active carbon and/or 10 ing. chemical reaction with e.g. hydrogen peroxide. According to CA 1 ,178,239, a process for the pro- The invention duction of sodium chlorate is combined with a membrane cell for the production of chlor-alkali. In this The invention relates to a process by which alkali case, the object is to simultaneously produce sodium 15 metal chlorate can be produced, whereby a number hydroxide, chlorine, hydrochloric acid and sodium of purification steps used in conventional processes chlorate, the chemicals needed in the cellulose pro- becomes superfluous. The essential features of the duction. In the chlorate electrolyser and membrane invention are the subject-watter of in dependant claim cell, aqueous chloride electrolyte is electrolyzed, 1, preferred features are the subject-watter of de- which electrolyte is obtained by addition of sodium 20 pendant claims 2-10. chloride to depleted chloride electrolyte from the According to the invention it relates to an integrat- anode compartment of the membrane cell. The con- ed process where the main part of the purification in centrated chloride electrolyte is purified in two steps, the process is made by precipitation, ion exchange where the content of Ca2+, Mg2+ and SO42" are re- and evaporation of the technical salt fed and dis- duced in the first step by precipitation. The contents 25 solved in water. By utilizing the condensate from the of Ca2*, Mg2+, Ba2* and the heavy metals are further chlorate crystallizer and the alkali metal chloride sub- reduced in a second step, by contacting the solution stantially purified for the production of chlorate, it is with an ion-exchange resin. The method for precipi- possible to produce very pure alkali metal hydroxide tation of chlorate is not evident from the patent. Use and chlorine and hydrogen chloride, respectively. of the produced sodium hydroxide in the production 30 These chemicals can be added to the chlorate proc- of sodium chlorate, is not mentioned either in the pa- ess without additional purification, as opposed to the tent. auxiliary chemicals being produced separately from According to US 3,897,320, chlorine and alkali the chlorate process. Especially, the invention con- metal hydroxide are produced in a membrane cell by cerns use of the alkali metal hydroxide produced in electrolysis of an aqueous alkali metal chloride solu- 35 the alkalization of chlorate electrolyte prior to the tion. The chlorine and chlorate-containing anolyte in crystallization of produced chlorate, in the hydrogen the membrane cell, is transferred to a chlorate cell for and reactor gas scrubbers and also in the precipita- further electrolysis to chlorate, which is precipitated in tion of impurities and regeneration of ion-exchange a crystallizer. The remaining mother liquor is returned resins in connection with dissolution and purification to the membrane cell by way of the arrangement for 40 of the technical alkali metal chloride. the production of fresh alkali metal chloride solution The advantage of the present process is besides ordirectly tothe chlorate cell. It is not evident from the the simple purification process also the large flexibil- patent, if the precipitation of chlorate takes place by ity as regards the amount of alkali metal hydroxide cooling or evaporation with direct or indirect condens- produced in relation to alkali metal chlorate. In the er. In this combined process, conventionally purified 45 production of hydrogen chloride, an excess of hydro- water and alkali metal chloride are used without any gen is desirable, which is achieved by using a com- special purification step, which makes it necessary to bination of hydrogen formed in the chlor-alkali proc- withdraw contaminated products such as chlorate ess and in the chlorate cells. Furthermore, the haz- and alkali metal hydroxide, so that the contents of im- ards of transporting primarily chlorine are reduced, purities can be controlled. Thus, this process is not 50 since the auxiliary chemicals are produced in immedi- useful when the process is closed to a high extent or ate connection to the location of consumption. Also, when pure products are required. The alkali metal hy- this means that a chlorine condensation plant is not droxide produced is preferably used in cooking and needed since the gaseous chlorine is immediately bleaching of groundwood pulp. consumed. Thus, various methods have been proposed to 55 According to the present process, the alkalized keep the concentration of impurities in the chlorate chlorate electrolyte is evaporated, which means that process at an acceptable level. Common to these water is removed by evaporation whereby the con- methods is either expensive purification of the raw centration of chlorate is increased to such an extent 3 5 EP 0 498 484 B1 6 that crystals precipitate. The water is recondensed by subsequent alkalization and 10-20% in the purifica- cooling in a heat exchanger. The water obtained as a tion of fresh brine. Alkalization relates to an increase condensate in the present process contains very low in the pH to a value above about 7. Suitably, the elec- contents of impurities, e.g. S042", due to the indirect trolyte in the crystallizer has a pH in the range from cooling. Furthermore, the amount of water being con- 5 about 8.5 to about 1 1 . densed in the chlorate crystallizer is sufficient for the The amount of alkali metal hydroxide produced in total requirement of water in the chlor-alkali process. the membrane ordiaphragm cell per ton of alkali met- Suitably, all of the water added to the membrane or al chlorate produced and recalculated as sodium hy- diaphragm cell for the production of alkali metal hydroxide per ton of dry sodium chlorate, can lie in the droxide, is taken from the evaporation of chlorate 10 range from about 10 to about 100 kg, suitably in the electrolyte. Preferably, the water separated in the range from 15 to 60 kg and preferably in the range crystallizer is brought to the cathode compartment of from 20 to 50 kg. It is especially preferred that the the membrane or diaphragm cell and/or to the prep- amount of alkali metal hydroxide produced, is essen- aration of electrolyte containing alkali metal chloride, tially equivalent to the amount of hydroxide used in which electrolyte is used to produce alkali metal hy- 15 the electrolysis of alkali metal chlorate. droxide by electrolysis. It is especially preferred to The chlorine produced in the anode compart- bring the water separated in the crystallizer to the ment of the membrane ordiaphragm cell, can be used cathode compartment. Suitably, the water separated for acidification in the production of alkali metal chlor- in the crystallizer is used for all requirements of water ate. Especially, the electrolyte fed to the chlorate in the process, such as for the preparation of electro- 20 electrolysers can be provided with chlorine, by ab- lyte containing alkali metal chloride which electrolyte sorption in water or directly in electrolyte. However, it is used to produce alkali metal chlorate by electroly- is preferred that chlorine is reacted with the hydrogen sis, forthe dilution of alkali metal hydroxide produced, formed in the cathode compartment of the mem- for the production of hydrochloric acid by absorption brane or diaphragm cell and/or the chlorate electro- of hydrogen chloride after an optional hydrogen chlor- 25 lysers, to hydrogen chloride. ide burner, in the scrubber liquids for purification of For an efficient process, a certain excess of hy- hydrogen and reactor gas, forthe washing of chlorate drogen is required. Preferably, said hydrogen can be crystals and optionally also for the dissolution of the taken from the chlorate electrolysis. The hydrogen technical salt fed for the preparation of alkali metal- chloride formed is absorbed in water for the produc- containing brine. 30 tion of hydrochloric acid, preferably in water from the In the electrochemical cell for the production of condenser of the chlorate crystallizer. The hydrochlo- chlorine, alkali metal hydroxide and hydrogen, the ric acid produced, is suitably used for acidification in anode and cathode compartments are suitably sepa- the production of alkali metal chlorate, especially of rated by a membrane or diaphragm mainly resistent electrolyte fed to the chlorate electrolysers. The addi- to chlorine and alkali metal hydroxide, preferably a 35 tion of hydrochloric acid and/or chlorine can be made membrane. Diaphragm relates to gas separating con- to any of the flows fed to the preparation of electrolyte structions of mainly inorganic material, such as as- forthe chlorate electrolysis, e.g. recirculating mother bestos, but also organic material, such as fluorine- liquorfrom the chlorate crystallizerand depleted elec- containing polymers, can be included. Membrane re- trolyte from the reaction vessels. Preferably, the ad- lates to ion selective, organic material, such as vari- 40 dition is made between the heat exchangers for cool- ous plastics and polymers. Suitably, the water to the ing of the electrolyte and the electrolysers, by absorb- cathode compartment is added from the indirect con- ing chlorine in a circulating portion of the flow of elec- denser of the chlorate crystallizer. trolyte, or by direct addition of hydrochloric acid to the The alkali metal hydroxide produced in the mem- main flow. In this position, the temperature is about brane or diaphragm cell, is used for alkalization of the 45 60 to about 70°C, which is suitable for absorption of chlorate electrolyte before the crystallizerand also for chlorine and the flow more than sufficient. Further- the precipitation of hydroxides of alkaline earth met- more, the residence time in the subsequent electro- als, iron and aluminium and regeneration of ion-ex- lysers means a further absorption potential at the change resins in the first and second step, respec- same time as the connection to the reactor scrubbers tively, of the purification of fresh brine. The alkali met- so meansan efficientway to take care of the chlorine not al hydroxide is used also in the hydrogen and reactor absorbed after all. In the electrolyte fed to the elec- gas scrubbers to remove chlorine in hydrogen from trolysers, suitably the pH lies in the range from about the chlorate cells and in the residual gas from optional 5.0 to about 7.5, preferably in the range from 6.5 to hydrogen chloride burners and for purification of 7.3. process air from the reaction vessels and optional 55 The amount of chlorine produced in the mem- chlorine absorption, respectively. Of the total amount brane or diaphragm cell, is stoichiometrically equiva- of alkali metal hydroxide, about 50% is used in the lent to the amount of alkali metal hydroxide produced. scrubbers, 30-40% in electrolyte filtration and the Consequently, the amount of chlorine produced per 4 7 EP 0 498 484 B1 8 ton of alkali metal chlorate produced recalculated as tained, with a concentration of from 290 to 310 g of dry sodium chlorate, can lie in the range from about sodium chloride/litre, is brought to a purification step 8.9 to about 89 kg, suitably in the range from 1 3 to 54 (2) for the precipitation of metals such as alkaline kg and preferably in the range from 18 to 45 kg. It is earth metals, iron and aluminium, by treatment with especially preferred that the amount of chlorine and 5 sodium hydroxide from the membrane cell and so- hydrogen chloride produced, is essentially equivalent dium carbonate. After sedimentation of the precipi- to the amount being used in the production of alkali tate, the brine is brought, by way of filtration, to a che- metal chlorate. late ion exchanger (3) for further purification. The The present process is suitably used in the pro- chelate ion exchanger is regenerated with sodium hy- duction of sodium or potassium chlorate, preferably 10 droxide from the membrane cell. Water is evaporated sodium chlorate, but other alkali metal chlorates can in a salt evaporator (4) and mechanical water separa- also be produced. The production of potassium chlor- tor. Said water is returned to (1). The main part, about ate can take place by addition of a purified solution of 90%, of the thus purified salt slurry is used for the potassium chloride to an alkalized part of the flow of preparation of electrolyte (6) for the production of sodium chlorate produced electrolytically, followed by 15 chlorate, together with chlorate electrolyte from the precipitation of crystals by cooling and evaporation. reaction vessels (9), mother liquor from the chlorate The chlorate is suitably produced by a continuous crystallizer (12) and transfer of chlorine-containing process, but a batchwise process is also useful. and acidic electrolyte from the membrane cells (15). When carrying out the process according to the The thus concentrated electrolyte contains from 100 present invention, an alkali metal chloride solution is 20 to 140 g of sodium chloride/litre and from 500 to 650 brought to an electrolyser with monopolar or bipolar g of sodium chlorate/litre, preferably from 110 to 125 cells. In a monopolar cell, the electrodes are connect- g of sodium chloride/litre and from 550 to 580 g of so- ed in parallel, which gives a high current intensity and dium chlorate/litre. The electrolyte is cooled to about low voltage drop. In bipolar cells, the anode of one cell 70°C and pH regulated (7) to within the range from 5.5 is connected to the cathode of the next, i.e. they are 25 to 6.5, whereupon the electrolyte is brought to the connected in series, which gives a low current inten- chlorate cells (8). The total flow to the chlorate cells sity and high voltage drop. The anode can be a met- is from 75 to 200 m3 of electrolyte per ton of sodium allic anode comprising a titanium base and a coating chlorate produced. Each chlorate cell works at a cur- of at least one of the metals of the platinum group or rent density of from about 10 to about 45 A/litre of cir- an oxide thereof being attached to the base. The 30 culating electrolyte. A portion of the chlorate electro- cathode can be produced from iron, carbon steel, lyte is recirculated to (6), while the other part, from stainless steel or titanium, or comprising such a metal about 1 5 to about 25%, is brought to (9) where the re- and a metal of the platinum group. action for the formation of chlorate continues. A por- The yield of chlorate is reduced and the energy tion of the flow from the reaction vessels, about 10%, consumption increased, by several reactions compet- 35 is electrolyte filtered completely or partly and alkal- ing with the desired formation of chlorate. The most ized by introducing a water solution of sodium hydrox- important of these is the cathodic reduction of CIO" ide from the membrane cells into a recirculating por- to CI", which reduction is counteracted by addition of tion of the f lowfrom the reaction vessels (9). The con- sodium dichromate. Suitably, the concentration of so- centration of sodium hydroxide in the water solution dium dichromate lies within the range from 1 to 6 g/li- 40 is suitably from 1 0 to 40 percent by weight, preferably tre of electrolyte and preferably within the range from from 25 to 35 percent by weight. The thus alkalized 3 to 5 g/litre of electrolyte. flow is brought to (12), while the rest of the reaction In the present process, use is made of different solution depleted in alkali metal chloride is returned temperatures in different steps, to facilitate e.g. the to (6) or directly by way of (7) to (8). The crystallizer absorption of chlorine, crystallization of alkali metal 45 has an indirect condenser (13), from which con- chlorate and desired electrochemical reactions. Suit- densed water with low contents of impurities is ably, the temperature in the electrolyte in the chlorate brought to (6), preparation of electrolyte (14) for the cells is from about 60 to about 90°C, preferably from production of chlorine and sodium hydroxide, to the 60 to 80°C. cathode compartment of the membrane cell (15) and The invention is now being described with refer- so to the dilution of sodium hydroxide withdrawn from ence to Figure 1, showing a schematic description of the cathode compartment, absorption of hydrogen a plant to produce sodium chlorate according to the chloride (17) and also to the scrubbers for hydrogen invention. Furthermore, the production of chlorine (18) and residual gas from the reaction vessels (19). and sodium hydroxide in a membrane cell is descri- In the crystallizer, the reaction solution is concentrat- bed, but the use of a diaphragm cell is also suitable. 55 ed by evaporation, whereby sodium chlorate crystal- Sodium chloride in the form of technical salt and lizes and is withdrawn by way of a filter. The mother recirculated water from the salt evaporator is brought liquor being saturated with respect to chlorate and to a dissolver (1). The close to saturated brine ob- containing high contents of sodium chloride, is re- 5 g EP 0 498 484 B1 10 turned to (6). Aminor part, about 10%, of the salt/salt brane cell, according to the present process. 20 kg of slurry purified in (1), (2), (3) and (4), is used for the sodium hydroxide/ton of sodium chlorate is required preparation of electrolyte (14) for the production of for alkalization, which is equivalent to 300 kg of so- chlorine and sodium hydroxide, by dissolution in wa- dium hydroxide/hour and 266 kg of chlorine/hour, or, ter or in recirculating electrolyte from/to the mem- 5 if chlorine is burned together with hydrogen to hydro- brane cell. The content of sodium chloride in the ano- gen chloride, 274 kg of hydrogen chloride/hour. In the lyte withdrawn, has dropped from about 300 to about example below, hydrogen chloride is used. 200 g of sodium chloride/litre, due to the electrolysis. Totally, 8,239 kg of sodium chloride/hour is con- The thus concentrated electrolyte contains from 250 sumed, of which 889 kg/hour is consumed in the to 300 g of sodium chloride/litre and from 1 to 4 g of 10 membrane cells. Of these, 439 kg/hour is consumed chlorine/litre, preferably from 270 to 300 g of sodium in the production of chlorine and sodium hydroxide chloride/litre and from 1 to 2 g of chlorine/litre. and the rest, 450 kg/hour, is transferred to the chlor- The sodium hydroxide formed in the cathode ate cells by way of 20% of the recirculated chlorine- compartment of the membrane cell, has after remov- containing anolyte. The content of sodium chloride is al from the cell a concentration of from about 10 to 15 250 g/litre and the flow 1 .8 m3/hour. about 40 percent by weight, preferably from 25 to 35 The total consumption of water is 7,606 kg/hour, percent by weight. A portion of the removed, highly divided into 2,489 kg/hour in the membrane cells and purified sodium hydroxide is diluted with water from 5,117 kg/hour in the chlorate cells. Of the 2,489 kg- the condenser or is brought undiluted to the electro- /hour which is consumed in the membrane cells, 700 lyte filtration (10), the alkalization step (11), the 20 kg/hour is part of the 30% strong sodium hydroxide scrubber system for hydrogen and reactor gas (18 leaving the cells. Furthermore, 1,620 kg H20/hour and 19, respectively), the step for precipitation of im- leaves the membrane cells byway of the 20% strong purities (2) and alsothe regeneration of ion-exchange share of the anolyte which is added to the chlorate resins for technical alkali metal chloride (3) and proc- cells and the remaining 135 kg/hour is consumed in ess water fed (5). The electrolyte from (11) is evapo- 25 the electrolysis. 605 kg of water/hour is consumed in rated in (12), whereby sodium chlorate crystallizes the absorption of hydrogen chloride. and the water evaporated is condensed in (13). After dewatering, the dry content of the crystalline sodium chlorate lies within the range from about 0.5 to about Claims 4 percent by weight, preferably within the range from 30 about 1 .5 to about 3 percent by weight. The very pure 1 . A process for the production of alkali metal chlor- water obtained, is brought to the preparation of elec- ate by electrolysis of an electrolyte containing trolyte (14) containing sodium chloride forthe produc- purified alkali metal chloride, in which a portion of tion of chlorine and sodium hydroxide, and also to the the flow of chlorate electrolyte obtained is alkal- cathode compartment of the membrane cell (1 5). The 35 ized and evaporated in a crystallizer whereby water and sodium hydroxide as well as the electrolyte chlorate precipitates, whereupon water evaporat- are so pure, that further purification steps are super- ed in the crystallizer is condensed, character- fluous. ized in that said condensed water and purified al- Suitably, chlorine formed in the anode compart- kali metal chloride prepared by dissolution and ment and hydrogen formed in the cathode compart- 40 purification of technical alkali metal chloride are ment, are brought to a hydrogen chloride burner (16) electrolysed in a membrane ordiaphragm cell for together with hydrogen formed in the chlorate cells, the production of chlorine and alkali metal hy- whereby hydrogen chloride formed is absorbed (17) droxide, which alkali metal hydroxide is used in in water from the condenser of the crystallizer and the production of alkali metal chlorate. added to the electrolyte forthe production of chlorate 45 immediately before the electrolysers (7). Unreacted 2. A process according to claim 1 , characterized in chlorine is absorbed in alkaline solution in the hydro- that all of the water added to the membrane or gen scrubbers (18), where hydrogen from the chlor- diaphragm cell for the production of alkali metal ate electrolysers is also purified. hydroxide, is taken from the evaporation of chlor- It is also quite possible to use chlorine directly, by 50 ate electrolyte. absorption in a circulating portion of the electrolyte flow for the production of chlorate (7). Chlorine not 3. A process according to claim 1 , characterized in absorbed, is brought to the reactor gas scrubbers that the amount of alkali metal hydroxide pro- (19), forabsorption in alkaline solution before purified duced, is essentially equivalent to the amount be- process air is vented to the atmosphere. 55 ing used in the production of alkali metal chlorate. Example: Electrolytical production of 15,000 kg of sodium chlorate/hour in combination with the 4. A process according to claim 1 or 3, character- equivalent amount of sodium hydroxide in a mem- ized in that the alkali metal hydroxide produced, 11 EP 0 498 484 B1 12

is used forthe alkalization of chlorate electrolyte wird. before crystallization of chlorate, in hydrogen- and reactor gas scrubbers as well as for the pre- 2. Verfahren nach Anspruch 1, dadurch gekenn- cipitation of impurities and regeneration of ion- zeichnet, daft das gesamte zu der Membran oder exchange resins in connection with dissolution 5 Diaphragmazelle zugegebene Wasser zur Her- and purification of technical alkali metal chloride. stellung von Alkalimetallhydroxid von der Ver- dampfung des Chloratelektrolyten entnommen 5. A process according to claim 1 , characterized in wird. that chlorine produced in the membrane or diaphragm cell, is used for acidification in the pro- 10 3. Verfahren nach Anspruch 1, dadurch gekenn- duction of alkali metal chlorate. zeichnet, daft die hergestellte Alkalimetallhydro- xidmenge im wesentlichen der Menge, die zur 6. A process according to claim 1 , characterized in Herstellung von Alkalimetallchlorat verwendet that chlorine produced in the membrane or dia- wird, Equivalent ist. phragm cell, and hydrogen formed in the mem- 15 brane or diaphragm cell and/or in the chlorate 4. Verfahren nach Anspruch 1 oder 3, dadurch ge- electrolysers are reacted to hydrogen chloride. kennzeichnet, daft das hergestellte Alkalimetall- hydroxid zur Alkalisierung von Chloratelektrolyt 7. A process according to claim 1 and 6, character- vor der Kristallisierung von Chlorat, in Wasser- ized in that hydrochloric acid produced by ab- 20 staff- und Reaktorgaswaschern sowie zur Fal- sorption of hydrogen chloride in water, is used for lung von Verunreinigungen und Regenerierung acidification in the production of alkali metal von lonenaustauscherharz in Verbindung mitder chlorate. Losung und Reinigung von technischem Alkali- metallchlorid verwendet wird. 8. A process according to claim 1 , 5, 6 or 7, charac- 25 terized in that the amount of chlorine and hydro- 5. Verfahren nach Anspruch 1, dadurch gekenn- gen chloride produced, is essentially equivalent zeichnet, daft das in der Membran oder Dia- to the amount being used in the production of al- phragmazelle hergestellte Chlor zur Ansauerung kali metal chlorate. bei der Herstellung von Alkalimetallchlorat ver- 30 wendetwird. 9. A process according to claim 1 , characterized in that the electrolyser for the production of chlor- 6. Verfahren nach Anspruch 1, dadurch gekenn- ine, alkali metal hydroxide and hydrogen is a zeichnet, daft das in der Membran oder Dia- membrane cell. phragmazelle hergestellte Chlor und der in der 35 Membran oder Diaphragmazelle und/oder in den 1 0. A process according to claim 1 , characterized in Chloratelektrolysezellen gebildete Wasserstoff that the electrolyser for the production of chlor- zu Chlorwasserstoff umgesetzt wird. ine, alkali metal hydroxide and hydrogen is a diaphragm cell. 7. Verfahren nach Anspruch 1 und 6, dadurch ge- 40 kennzeichnet, daft die durch Absorption von Chlorwasserstoff in Wasser hergestellte Salz- Patentanspruche saure zur Ansauerung bei der Herstellung von Al- kalimetallchlorat verwendet wird. 1. Verfahren zur Herstellung von Alkalimetallchlorat durch Elektrolyse eines gereinigtes Alkalimetall- 45 8. Verfahren nach Anspruch 1, 5, 6 oder 7, dadurch chlorid enthaltenden Elektrolyten, wobei ein Teil gekennzeichnet, daft die hergestellte Chlor- des erhaltenen Chloratelektrolytflusses alkali- menge und Chlorwasserstoffmenge imwesentli- siert wird und in einem Kristallisierereingedampft chen Equivalent der Menge, die zur Herstellung wird, wodurch Chlorat ausfallt, wahrend von Alkalimetallchlorat verwendet wird, ist. verdampftes Wasser im Kristallisierer konden- 50 siert, dadurch gekennzeichnet, daft das kon- 9. Verfahren nach Anspruch 1, dadurch gekenn- densierte Wasser und durch Losung und Reini- zeichnet, daft die Elektrolysezelle zur Herstel- gung von technischem Alkalimetallchlorid herge- lung von Chlor, Alkalimetallhydroxid und Wasser- stelltes gereinigtes Alkalimetallchlorid in einer staff eine Membranzelle ist. Membran- oder Diaphragmazelle zur Herstellung 55 von Chlor und Alkalimetallhydroxid elektrolysiert 10. Verfahren nach Anspruch 1, dadurch gekenn- werden, wobei das Alkalimetallhydroxid bei der zeichnet, daft die Elektrolysezelle zur Herstel- Herstellung von Alkalimetallchlorat verwendet lung von Chlor, Alkalimetallhydroxid und Wasser- 7 13 EP 0 498 484 B1 14

stoff eine Diaphragmazelle ist. I'eau, est utilise pour I'acidification dans la production de chlorate de metal alcalin.

Revendications 8. Precede selon la revendication 1, 5, 6 ou 7, ca- 5 racterise en ce que la quantite de chlore et de 1 . Pro-cede pour la production de chlorate de metal chlorure d'hydrogene produit est essentiellement alcalin par electrolyse d'un electrolyte contenant equivalente a la quantite a utiliser dans la produc- du chlorure de metal alcalin purifie, dans lequel tion de chlorate de metal alcalin. une partie du debit de I'electrolyte de chlorate ob- tenu est alcalinise et evapore dans un cristallisoir w 9. Precede selon la revendication 1 , caracterise en d'ou le chlorate precipite, ce sur quoi I'eau eva- ce que I'electrolyseur pour la production de chlo- poree est condensee dans le cristallisoir, carac- re, d'hydroxyde de metal alcalin et d'hydrogene terise en ce que ladite eau condensee et ledit est une cellule a membrane. chlorure de metal alcalin purifie prepare par dissolution et purification du chlorure de metal alca- 15 10. Precede selon la revendication 1, caracterise en lin technique sont electrolyses dans une cellule a ce que I'electrolyseur pour la production de chlo- diaphragme ou a membrane pour la production re, I'hydroxyde de metal alcalin et I'hydrogene est de chlore et d'hydroxyde de metal alcalin, lequel une cellule a diaphragme. metal alcalin est utilise dans la production de chlorate de metal alcalin. 20

2. Precede selon la revendication 1, caracterise en ce que toute I'eau ajoutee a la cellule a diaphragme ou a membrane pour la production d'hydroxyde de metal alcalin, provient de I'evaporation de 25 I'electrolyte de chlorate.

3. Precede selon la revendication 1, caracterise en ce que la quantite d'hydroxyde de metal alcalin produit est essentiellement equivalents a la 30 quantite utilisee dans la production de chlorate de metal alcalin.

4. Precede selon la revendication 1 ou 3, caracterise en ce que I'hydroxyde de metal alcalin produit 35 est utilise pour I'alcalinisation de I'electrolyte de chlorate avant la cristallisation du chlorate, dans des epurateurs d'hydrogene et de gaz de reac- teur aussi bien que pour la precipitation des im- pure tes et la regeneration des resines echangeu- 40 ses d'ions en relation avec la dissolution et la purification du chlorure de metal alcalin technique.

5. Precede selon la revendication 1, caracterise en ce le chlore produit dans la cellule a membrane 45 ou le diaphragme est utilise pour I'acidification dans la production de chlorate de metal alcalin.

6. Precede selon la revendication 1, caracterise en ce que le chlore produit dans la cellule a dia- 50 phragme ou a membrane, et I'hydrogene forme dans la cellule a diaphragme ou a membrane et/ou dans les electrolyseurs de chlorate reagis- sent en chlorure d'hydrogene. 55 7. Precede selon les revendications 1 et 6, caracterise en ce que I'acide chlorhydrique produit par absorption du chlorure d'hydrogene dans de 8 EP 0 498 484 B1