FI111707B - Process for preparing concentrated alkali metal hypochlorite solutions - Google Patents

Abstract

The invention relates to a process for the preparation of concentrated alkali metal hypochlorite solutions in which chlorine and the alkali metal hydroxide are injected into a vertical receptacle, the bottom part of which has a cross-section less than the cross-section of its top part. This receptacle is equipped with an external hypochlorite recycling loop.

Description

111707

The present invention relates to a process for the preparation of concentrated alkali metal hypochlorite solutions. The present invention relates to a process for the preparation of concentrated alkali metal hypochlorite solutions. In particular, the invention is used in a concentrated hypochlorite solution having a concentration of more than 100 degrees of chlorometry and under-saturated with NaCl. The preparation of the hypochlorite solution is accomplished by reacting the chlorine with the sodium hydroxide solution and producing as many moles of hypochlorite as moles of sodium chloride. After a certain concentration, part of the NaCl precipitates. In the preparation of concentrated hy-10 chlorite solution, NaCl is always present in a saturated form. Since the solutions are not stable over time, NaCl crystals are continuously formed as a result of degradation reactions. These salt crystals cloud the solution and disintegrate. In addition, in certain applications, the presence of salt is harmful.

FR Patent 1352198 proposes the preparation of concentrated hypochlorite solutions and then dilution thereof to obtain more stable solutions. This patent also describes an apparatus for the preparation of this concentrated hypochlorite solution consisting of a cylindrical vertical column with a conical bottom and an external recirculation system equipped with a pump and a heat exchanger. The production of the hypochlorite solution is recovered; v, from the top of the 20 columns, the hypochlorite solution is withdrawn from the top of the column to circulate! ! heating system. Once the hypochlorite solution has run in the pump and heat exchanger, it is recycled to the conical bottom of the column. From the bottom of the column, feed: · 'chlorine and sodium hydroxide and remove the crystalline solution in the hypochlorite solution'. ': NaCl suspension. The fluid velocity in the column is adjusted so that the NaCl crystals are suspended. The 25 pence remains in a fluidized state and that the precipitation of NaCl occurs mainly through crystal growth. Large particles settle on the conical bottom of the column and • are therefore easy to remove. In order to prevent the formation of fine particles' on a large scale, it is sufficient for the fluid bed to fill the entire reaction volume. If the fluid bed is not of sufficient volume, it will be difficult to form: 30 separable fines if the fluid bed is too large in volume,

The NaCl crystals are removed with the production water of the hypochlorite solution. It's a must- · ,. also push to start feeding the suspension of sodium chloride crystals having a suitable starting particle size.

Also known in the prior art is U.S. Patent 4,428,918 for vertical co-35 columns provided with external pump-driven recirculation via a heat exchanger. The hypochlorite solution is withdrawn from the top of the column via a back-recycle system. Once the hypochlorite solution has passed through the pump and trap, it is recycled to the bottom of the column. Sodium hydroxide and chlorine are also sprayed from the bottom of this column, but in contrast to the prior art mentioned above, this column is fitted with a chimney that enables an internal gas-siphon-type circulation under the influence of chlorine feed rate in addition to the circulation generated by the external recirculation pump. The produced hypochlorite water is withdrawn from the top of the column, which must then be filtered to remove the NaCl crystals to give a concentrated hypochlorite 10 solution. In fact, contrary to prior art, in FR patent 1352198, complete mixing takes place on the entire column, which thus contains a suspension of NaCl crystals in concentrated hypochlorite. Filtration is very difficult because the crystals are not large enough and their size varies greatly.

U.S. Patent No. 4,780,303 also proposes a continuous two-step process in two separate installations. The first step is stopped at a NaClO concentration such that NaCl does not precipitate, then in the second step chlorine and sodium hydroxide are added again, the NaCl crystals are precipitated and the concentrated hypochlorite solution is recovered from the top of the column. The NaCl crystals are removed from the bottom of this second stage column. This method can indeed produce continuous-functionally concentrated hypochlorite which no longer contains crystals. In contrast, this method is expensive in terms of investment and also in terms of maintenance, since the reaction equipment is expensive. · There are two and both equipped with a pump and a heat exchanger.

A much simpler method has now been invented for the concentration of concentrated alkali metal hypocyclo; for the preparation of lithium solutions. The invention relates to a process for the preparation of a concentrated solution of an alkali metal hypochlorite by reacting a solution of chlorine and the corresponding alkali metal hydroxide in the presence of crystals of alkali metal chloride, characterized by a substantially vertical vessel having a lower part than (a) injection of chlorine into the lower portion and an alkali metal hydroxide solution, (30) (b) removal of the hypochlorite solution from the upper portion, (i) production of a concentrated hypochlorous solution, (ii) recycling of the lower portion to the lower portion; (a) adjust the reagent recycling and injection so that the alkali metal chloride crystals are substantially suspended in the lower part of the vessel.

3, 111707

The essential features of the invention are set forth in the appended claims.

Pure liquid or gaseous chlorine or a gas containing chlorine and inert to the reaction may be used; for example, air or nitrogen with added chlorine. The concentration of the hydroxide solution is chosen according to the concentration of the hypochlorite solution to be prepared. Concentrated hydroxide solution may also be used and water may be added. The reaction is complete and stoichiometric. It is possible to operate at any pressure, but it is much simpler to operate at normal pressure. The vessel may be open, but preferably is closed and connected to an absorptive system to ensure that, in the event of an accident, chlorine is not released into the atmosphere, although at all times using a small amount of hydroxide. The temperature of the solution inside the vessel and in the recycling system is kept below 35 ° C to prevent chlorate formation. Preferably, the heat exchanger may be fitted to a recycling system, or may be fitted to a container, or use a container having a double jacket to circulate a low temperature fluid, or a combination of all of these means.

The vessel may be a simple column with a second column of larger diameter and the columns interconnected in the form of a truncated cone. The ratio of the cross section of the upper to the cross section of the lower should be sufficient for the Leu-entrained crystals to remain in the lower bulk and, if too high, to decant to the upper and lower back to the lower.

: '. Thus, growth of alkali metal chloride crystals is obtained in the presence of chloride precipitation on existing crystals. Apparently, there are also new small crystals ··· 'that are present throughout the vessel and flow with the recycling. The cross-section ratio is preferably greater than 1.2 and may be from 2 to 12. The bottom volume may be at least 30% of the total volume of the container and preferably 40-60%. Bottom volume

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'·' Is determined, as in the prior art, by the reaction of chlorine with an alkali metal hydroxide.

> *, · ·. The part of the hypochlorite which is isolated in step b) and which forms the product can be / then filtered or slightly diluted to destroy small alkali metal chloride crystals. An advantage of the process according to the invention is that it can provide a very concentrated hypochlorite solution which can then be slightly diluted to completely eliminate even the smallest chloride crystals. An advantage of the process is that it produces a concentrated hypochlorite solution which is practically crystalline. Slight dilution then causes the last 35 NaCl crystals to disappear, but above all, it can produce a concentrated solution which is not saturated with alkali chloride. This solubility reserve prevents the salt crystals from reappearing before use. Thus, hypochlorite containing 27% active chlorine can be produced, which can always be diluted to a solution containing 25% active chlorine. To refresh the memory, the conversion of degrees of chlorometry to active Cl2 is as follows: 5 degrees of chlorometry x 3.17 g / l x 100% active Cl2 = density (g / l)

Chlorometry Degree Density% Active Cl2 115 1335 27.3 100 1315 24.1 10 Another advantage of this process, when used on an industrial scale, is that it does not need to add alkali chloride crystals to initiate crystallization during the start-up step. The fine particles in the reactor gradually grow to form a liquid layer.

According to a preferred embodiment of the invention, step b) is performed to remove the concentrated hypochlorite solution produced by the club from the top of the top and to recycle the hypochlorite solution at a location located at the top but below the outlet. The circulation and the injection of reagents can be controlled such that the small chloride crystals normally present at the top rise to where ·; the rag solution is removed but does not rise to the top of the top. Proficient in the art; '20 can easily determine this height, which separates the upper end of the upper part from the outlet of the recycle solution. It has been found that it is sufficient to place it between the half: top of the container.

Fig. 1 illustrates a possible embodiment of the present invention. The reference numerals 1 i · <and 2 refer to the lower and upper parts of the container respectively. The product-25 concentrated hypochlorite is removed from step 15, the hypochlorite solution is removed from step 10 for recycling. towards pump 4 and returns from position 11 to lower part 1 after passing through heat exchanger 3 t *. The alkali metal hydroxide solution is injected from tube 13 and chlorine from tube 12.

'·,; Alkali metal chloride is withdrawn from tube 14.

The present invention also relates to apparatus for carrying out the method of the invention. This apparatus consists of a substantially vertical vessel having a lower section smaller than its upper section and having: - a tube for recycling, injection of reagents and removal of an alkali metal chloride, - a tube for recovering a hypochlorite solution, recycling equipment and possibly a heat exchanger.

According to a preferred embodiment of the invention, there is a tube at the top of the top of the vessel for recovering the produced hypochlorite, and at the top, preferably halfway up the height of this top, an outlet for recycling.

Example

The apparatus of Fig. 1 having the following characteristics: - the reactor used consists of three parts: 10 - a conical lower part (1300, height = 1900) which receives the return of the cooling system - a central cylindrical part (1300, height = 3500) which and a fluidization zone; the reagents are injected from the lower part of this zone - the upper part of the cylindrical truncated cone (3500, height = 1900), from the so-called decantation zone; - the starting point of the cooling system is at the lower end of the cylindrical part while the finished product is removed from the upper part; The volume of the Λ-container is 15 m ',,, - the lower part makes up 37% of the container-volume', · 20 - the ratio of the cross-sections is 7, "3.

:: - recycling rate: 60 m / hour; the heat exchanger cools from 30 ° C to 20 ° C; '; injected: - 1270 kg / h NaOH (sodium 50%) - 1100 kg / h 97% chlorine 25 - 1200 kg / h water upstream of the recirculation pump.

,,. Receiving: t I 1 | | Λ • ,,; - 3 m '/ hour sodium hypochlorite solution containing 27% active Cl2' ·; · '- 600 kg / h NaCl crystals.

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After recovery, the hypochlorite solution is diluted with 27% active chlorine *; '30 to 25%.

, *,: Stability of the resulting hypochlorite solution: - The average daily loss over 7 days is about 0.35% active chlorine at 17 ° C.

The advantages of the process are: 6,111,707 - producing a concentrated hypochlorite solution (> 24% active chlorine), free of salt crystals and above all not saturated with salt - this solubility reserve prevents the crystals from reappearing before using the product (the hypochlorite solution no longer turbid and at the bottom of storage vessels or tank vehicles) - it is possible to use unrefined chlorine: for example, chlorine derived directly from electrolysis can be used without the inert gases it contains interfere with the control of salt crystallization 10 - no need to add NaCl crystals to initiate crystallization up phase.

The fine particles are injected back into the reaction zone.

The crystals gradually grow to form a fluidized bed.

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Claims (5)

A process for preparing a concentrated solution of an alkali metal hypochlorite by reacting chlorine and a corresponding alkali metal hydroxide solution in the presence of alkali metal chloride crystals, the method comprising introducing a chlorine and a hydroxide solution onto a bottom of a vessel, that in a vessel having a simple vertical conical column with another vertical column of larger diameter, the columns being connected together in the form of a truncated cone: 10 a) injecting chlorine and alkali metal hydroxide solution into the lower part, b) taking a hypochlorite solution from the upper part; (ii) recycling the second portion to the lower portion of the vessel, (c) removing the alkali metal chloride crystals near the lower portion of the vessel, (d) recycling the reagents from step (a) and adjusting the injection so that the alkali metal chloride crystals float substantially at the bottom of the vessel. Method according to Claim 1, characterized in that the ratio of the upper part of the vessel to the cross-section of the lower part is greater than 1.2 and preferably 2-12. Method according to claim 1 or 2, characterized in that the volume of the lower part 20 constitutes at least 30% of the total volume of the container and preferably 40-60%. Method according to one of Claims 1 to 3, characterized in that step b) is carried out by recovering the concentrated hypochlorite solution produced from the upper end of the upper part and recovering the hypochlorite solution at a location located at the upper part: 25 below the recovery point. ; A method according to claim 4, characterized in that the removal for recycling '·' takes place from the middle height of the top. 8 111707 1. Förfarande for framställning av en concentrerad alkali metal hypochloritelösning genome att bringa chlorochen motsvarande alkali metal hydroxide lösning att reagent i alkali metal chloride crystalline, förfarande bestär i att man inför 5 so chlor och enönen av alkali metal crystal crystal crystal crystal, av att man i en behallare bestäende av en angel vertical conical column anordnad över en giving vertical column med större section, color columns and förbundna med stock medelst en stympad kon 10 a) inför i det nedre partieth chlorine och en alkalimet b) i det övre partiet Uttar hypochloritelösningen, d) (i) en delguter denier concentratorade hypochloritelösningen och (ii) en annel delterter circulation i det nedre partiet av behällaren, (c) utmmermer crystalline av alkali metal chloride vid nedre delen av det nedreiet av behäll Aren, d) reglerar atercirculation and insertion of reagent reagents a) crystallization of alkali metal chlorides and fluidisation of interest; 'Behällaren. :: 2. Förfarande enligt patentkrav 1, kännetecknat av att förhällandet mellan sek- *: 20 thionen för det övre partiet av behällaren och sectionen för detre partiet översti-; '. \ ger 1,2 och att det företrädesvis uppgär account mellan 2 och 12. '3. Förfarande enligt patentkrav 1 eller 2, vouchernat av att volymen för det nedre partiet representerar 30% av den total volume för behällaren och fö-': reträdesvis 40-60%. * 25 4. Förfarande enligt nägot av patentkrav 1-3, kännetecknat av att man genomför: steget b) genom att utta denta bildade concentrerade hypochloritelösningen vid den övre: delen av det övre partiet och att man Uttar hypochloritelösningen för ätercirculationen vid et stára partiet men under uttagsstället. '· 5. Förfarande enligt patentkrav 4, kännetecknat av att uttagningen för 30 ätercirkulationen genomföres vid halöjden av den övre sektionen.