Sodium percarbonate and process for its production

0001The present invention relates to a novel sodium percarbonate of the general formula Na₂CO_3.1.5 H₂0₂.

0002The sodium percarbonate known to date is either:<ul><li>A hexagonal prism-shaped single crystal with a density of between 0.9 and 1 g / cm 3 (<patcit id="pcit0001" dnum="FR2318112"><text>FR 2 318 112</text></patcit>) is</li><li>A monocrystal in the form of a regular rhombohedron (<patcit id="pcit0002" dnum="FR2355774"><text>FR 2 355 774</text></patcit>) is</li><li>A hollow granule having a bulk density of about 0.4 g / cm 3 and an average diameter close to 480 μm (<patcit id="pcit0003" dnum="FR2486056"><text>FR 2 486 056</text></patcit>) is</li><li>In the form of a compact grain with an average grain size close to 450 μm (<patcit id="pcit0004" dnum="BE859155"><text>BE 859 155</text></patcit>).</li></ul>

0003A very high quality sodium percarbonate has now been found, consisting of small agglomerated crystals. The size of these small crystals is generally between about 1 and about 100 μm and preferably between about 5 and about 20 μm

0004The sodium percarbonate of the present invention consisting of agglomerates of small crystals of sodium percarbonate is characterized by a very good resistance to attrition, preferably with a weight loss in fine less than 1% according to the test of the ISO standard 5937 (fluid bed friability method).

0005The sodium percarbonate in the form of agglomerates according to the invention has a bulk density greater than about 0.6 g / cm 3 and preferably between about 0.75 g / cm 3 and about 1.1 g / cm 3.

0006The sodium percarbonate agglomerates of the invention have the advantage of dissolving rapidly. Thus, the time required to obtain the dissolution of 90% of 2 g of sodium percarbonate agglomerates introduced into a liter of water is generally less than or equal to 90 seconds at 15 ° C. and preferably between 40 and 70 seconds .

0007The sodium percarbonate according to the invention is formed from agglomerates whose average particle size varies with the conditions chosen for their manufacture.

0008The latter described below make it possible to obtain agglomerates whose particle size can be between limits as far as about 160 μm and approximately 1400 μm with a particularly narrow particle size distribution. For example, in the case of agglomerates with an average particle size equal to approximately 765 μm, generally at least 85%, have a size of between approximately 480 μm and approximately 1400 μ<i>M</i>.

0009Advantageously, the average particle size of these agglomerates is greater than 600 μm<i>M</i> And preferably greater than 700 μ<i>M</i>.

0010The weight ratio of fines smaller than 160 μ<i>M</i> Present with the agglomerates as they result from the process which is suitable for their manufacture is less than 2%, more often even less than 1%.

0011The active oxygen content of these sodium percarbonate agglomerates is generally greater than 14% and is defined as the percentage by mass of the amount of available oxygen relative to the sodium percarbonate agglomerates.

0012The percarbonate of the present invention can be obtained by a process for the continuous manufacture of sodium percarbonate in the form of agglomerates consisting of small crystals and having at least one of the characteristics described above. It is known to produce sodium percarbonate by reacting a solution or suspension of sodium carbonate with aqueous solutions of hydrogen peroxide. The addition of inert salts such as sodium chloride to lower the solubility of sodium percarbonate is also known.

0013On the other hand, processes for the batch production of sodium percarbonate (<patcit id="pcit0005" dnum="BE859155"><text>BE 859 155</text></patcit>, <patcit id="pcit0006" dnum="FR2368438"><text>FR 2 368 438</text></patcit>). Methods are also known for the continuous manufacture of sodium percarbonate operating either under vacuum (<patcit id="pcit0007" dnum="FR2318112"><text>FR 2 318 112</text></patcit>) Or with two reactors in cascade (<patcit id="pcit0008" dnum="EP496430A"><text>EP 496 430</text></patcit>) And respectively leading to single crystals and crystals of sodium percarbonate having a size of less than 75 μm

0014The patent <patcit id="pcit0009" dnum="GB1469352A"><text>GB 1,469,352</text></patcit> Discloses a process for the production of abrasion-resistant coarse-grained sodium percarbate.

0015The patent <patcit id="pcit0010" dnum="US2986448A"><text>US 2,986,448</text></patcit> Suggests the use of a crystallizer, in which a supersaturated solution is carried upwardly through a bed of crystals of persalts in formation and growth, thus making it possible to achieve a certain classification of particles. However, the particle size distribution of the persalts thus obtained is still relatively spread and their stability is not very high.

0016The European patent application <patcit id="pcit0011" dnum="EP703190A"><text>EP 703 190</text></patcit> Describes a continuous process of making the persalts in a classifying crystallizer. Persels are formed in a vigorously agitated area (page 2, lines 1 to 2).

0017The Applicant has now discovered a process for the continuous manufacture of sodium percarbonate based on the generation and agglomeration of small crystals of said sodium percarbonate.

0018This process is characterized in that it comprises a reactor in which a bed of small crystals and / or agglomerates of sodium percarbonate is kept in suspension by an ascending current of a supersaturated aqueous solution of said percarbonate formed by Contact with a suspension or solution of sodium carbonate and an aqueous solution of hydrogen peroxide. The saturation state of the aqueous solution will decrease as it moves in a continuous upward motion at such a rate that a desired granulometric agglomerate classification is ensured.

0019The thus produced sodium percarbonate agglomerates are removed from the suspension in the lower part of the bed.

0020At the end of its upward movement, the aqueous solution is supersaturated with sodium percarbonate and has a concentration of sodium percarbonate generally between the value corresponding to the solubility of sodium percarbonate in the same medium at the same temperature and about 1 , 6 times this value. The aqueous solution at the end of its upward displacement generally has a sodium percarbonate concentration of between about 1 and about 1.4 times the value corresponding to the solubility of the sodium percarbonate in the same medium and at the same temperature . The amount of solid present at this reactor level is generally less than 150 g per liter of aqueous solution and preferably between about 10 and about 25 g per liter of aqueous solution.

0021In order to form the sodium percarbonate intended to ensure the supersaturated state of the supersaturated aqueous solution of sodium percarbonate, amounts of hydrogen peroxide and sodium carbonate are used such that the molar ratio of hydrogen peroxide to carbonate of Sodium dissolved in the mother liquors after the upward displacement of the supersaturated aqueous solution is greater than 1 and preferably between about 1.2 and about 1.6.

0022The addition of sodium percarbonate in an aqueous solution ensuring this state of supersaturation can be achieved by forming said percarbonate within or even externally from the supersaturated aqueous solution of percarbonate. The formation of sodium percarbonate in the supersaturated aqueous solution of sodium percarbonate is particularly preferred and is carried out by the continuous introduction of an aqueous solution of hydrogen peroxide and a suspension or solution of carbonate of Sodium, optionally containing sodium percarbonate in solution or suspension. Preferably, the aqueous solution of hydrogen peroxide is introduced into the ascension path of the supersaturated aqueous solution of sodium percarbonate at a level very close to that of the aqueous solution, Introduction of the sodium carbonate suspension or solution. The introduction of the aqueous solution of hydrogen peroxide and of the sodium carbonate suspension or solution can also take place at several levels.

0023Advantageously, the sodium carbonate suspension or solution is introduced into the zone lying between the level situated at the upper limit of the withdrawal of the agglomerates and the level situated approximately halfway through the ascending path of the supersaturated solution of sodium percarbonate.

0024The state of supersaturation of the aqueous solution of sodium percarbonate in the introduction zone of the hydrogen peroxide solution and the sodium carbonate suspension or solution is normally greater than 1.2 times the value of the solubility Sodium percarbonate in the same medium and at the same temperature. It is preferably between about 1.3 and about 6 times this value.

0025The generation of the sodium percarbonate crystals and / or agglomerates can be carried out in the presence of crystallizing agents. These agents may be introduced at one or more levels (x) in the upward path of the supersaturated aqueous solution of sodium percarbonate. These levels are preferably in the region of introduction of the aqueous solution of hydrogen peroxide and the sodium carbonate suspension or solution and / or above this zone. Of the crystallizing agents, sodium hexametaphosphate is particularly preferred. In general, an amount of crystallizing agents is used such that their concentration in the supersaturated aqueous solution of sodium percarbonate is greater than 0.1 g / l and most often between about 0.5 g / l and about 2, 7 g / l.

0026Anionic surfactants can also be used to control the crystallization of sodium percarbonate. These agents are preferably introduced at the zone of introduction of the suspension or solution of sodium carbonate. Anionic surfactants containing at least one sulfate or sulfonate function attached to a hydrocarbon chain are particularly preferred. A quantity of surfactants is generally used such that their concentration in the mother liquors is greater than 0.1 g / l and most often between about 0.7 g / l and 1 g / l. The isobutyl sulfate oleate is advantageously chosen from surfactants.

0027To reduce the solubility of sodium percarbonate, salting agents such as sodium salts can also be used. Sodium chloride is particularly preferred. The amount of release agents used is such that their concentration in the mother liquors is greater than 20 g per liter and preferably between about 70 g / l and about 170 g / l.

0028The continuous introduction of the hydrogen peroxide into the solid-liquid suspension is ensured by an aqueous solution of hydrogen peroxide having a concentration by weight of between about 35% and about 70%. The aqueous hydrogen peroxide solution may additionally contain sodium carbonate, stabilizing agents, in particular sodium silicate, magnesium sulphate and release agents such as sodium chloride.

0029Continuous introduction of sodium carbonate into the solid-liquid suspension is provided by a concentrated aqueous suspension or sodium carbonate solution, the titer of which is greater than 10% and preferably between about 15% and about 24% Optionally containing sodium percarbonate in solution or in suspension. The concentrated sodium carbonate solution may be prepared by dissolving the commercial sodium carbonate in water or in part or all of the solution removed from the solid-liquid suspension at the end of its upward displacement at a temperature above 17 ° C. The dissolution temperature is preferably from about 30 ° C to about 70 ° C.

0030Any type of sodium carbonate having an iron (Fe) content of less than 10 ppm may be suitable. Anhydrous sodium carbonate is preferably used from SOLVAY or RHONE-POULENC.

0031The suspension or concentrated solution of sodium carbonate may also contain stabilizing agents, such as, in particular, sodium silicate or magnesium sulphate, release agents such as sodium chloride and crystallizing agents such as sodium hexametaphosphate

0032The stirring state of the solid-liquid suspension in which the upward velocity of the liquid is involved is ensured by a paddle or propeller stirrer or a parrot ladder. This agitation must be such that the very state of suspension and the grading effect are ensured and that the small crystals of sodium percarbonate are kept sufficiently long in the contact position or in a sufficient proximity necessary for their agglomeration.

0033The linear upward velocity of the liquid in the cylindrical portion of the reactor can be adjusted from about 2 m / h to about 20 m / hr. Advantageously, a linear velocity of between about 3 m / h and about 10 m / h is used. By linear ascending velocity is meant the ratio between the fluidization rate and the section of the reactor.

0034The temperature of the solid-liquid slurry is from about 14 to about 20 ° C. It is precisely controlled by one or more heat exchangers in parallel. A temperature of the solid-liquid suspension close to 17 ° C. is particularly preferred and is advantageously controlled to within 1 ° C.

0035At the end of the upward displacement of the solid-liquid suspension, it is possible to separate the solid matter from the liquid by conventional techniques such as decantation, filtration or by using a hydrocyclone.

0036Some or all of this liquid, separated or not separated from the solid matter, from the mother liquors resulting from the separation of the manufactured agglomerates from the liquid removed with them, and possibly from water, normally constitute the liquid flow entering the bottom Reactor in which the agglomerates according to the invention are formed.

0037According to the present invention, it is possible to use a column reactor or a reactor of cylindroconical shape provided or not with a flange on its upper part. A cylindroconic reactor and, preferably, a cylindroconical reactor provided with a collar are most commonly used.

0038The <figref idrefs="f0001">figure 1</figref> Illustrates schematically an exemplary embodiment of the process for manufacturing the percarbonate of the invention.

0039On the <figref idrefs="f0001">Figure 1, 1</figref> Refers to a cylindroconical reactor and the agitator of which this reactor is provided; 3 and 14 denote the feeding into the reactor 1 of an aqueous solution of hydrogen peroxide containing the stabilizing agents 4 and 13 that of a sodium carbonate solution containing sodium silicate and optionally percarbonate in solution or suspension; 5 refers to the supply of anionic surfactant to the reactor 1, 6 as the crystallizing agent; 7 denotes the conduit through which the liquid optionally containing the solid leaves the reactor at the end of the ascent; 8 denotes the container in which the commercial sodium carbonate introduced through inlet 9 is dissolved in a portion of the liquid from line 7; 10 refers to the overflow tank, Another part of the liquid coming from the pipe 7 is retained before being introduced to the bottom of the reactor by the pipe 11. 12 denotes the outlet of the liquid containing the sodium percarbonate agglomerates. These agglomerates are then isolated from the liquid or mother liquors containing them by spinning using a centrifugal extractor and then dried in a fluidized bed at a temperature of between approximately 40 and approximately 70 ° C., preferably between approximately 50 and approximately 60 ° C. The mother liquors recovered at the outlet of the wringer can be reintroduced into the overflow tank 10. These agglomerates are then isolated from the liquid or mother liquors containing them by spinning using a centrifugal extractor and then dried in a fluidized bed at a temperature of between approximately 40 and approximately 70 ° C., preferably between approximately 50 and approximately 60 ° C. The mother liquors recovered at the outlet of the wringer can be reintroduced into the overflow tank 10. These agglomerates are then isolated from the liquid or mother liquors containing them by spinning using a centrifugal extractor and then dried in a fluidized bed at a temperature of between approximately 40 and approximately 70 ° C., preferably between approximately 50 and approximately 60 ° C. The mother liquors recovered at the outlet of the wringer can be reintroduced into the overflow tank 10.

Example 1

0040The procedure is carried out according to the <figref idrefs="f0001">figure 1</figref>.

0041The cylindroconical reactor 1 has a total height (cylindrical and conical parts) equal to 51 cm; Its cylindrical part has a height equal to 43 cm and a diameter equal to 10 cm; It is provided with a flange 13 cm high in its upper part (not shown in the diagram) and is mainly equipped with stirrer 2, injectors for introducing aqueous hydrogen peroxide solutions, Sodium carbonate, surfactants and crystallization.

0042The operation is initiated by introducing 760 g of sodium percarbonate crystals having a density of 0.75 g / cm 3, previously prepared, in reactor 1 containing a (% by mass) mother liquor:<ul><li>7.2% sodium carbonate</li><li>3.48% of a solution of 70% hydrogen peroxide</li><li>7.2% sodium chloride</li><li>0.13% of the isobutyl sulfate oleate</li><li>500 ppm sodium hexametaphosphate (PROLABO RECTAPUR)</li><li>495 ppm sodium silicate</li><li>99 ppm magnesium sulphate</li></ul>

0043The reactor 1 is then continuously introduced, stirred at 70 rpm, 3 and 14, in total approximately 550 cm³/ H of the aqueous solution of hydrogen peroxide (titer of 70%) containing 5% of sodium chloride and 70 ppm of magnesium sulphate, By 4 and 13 in total 6.1 l / h of the concentrated sodium carbonate solution was 17-19% by weight, By 5 - 12.4 g / h of isobutyl sulfate oleate, By 6 - 20 cm³/ H of the sodium hexametaphosphate solution was 24.7% by mass.

0044The flow rate of the aqueous solution of hydrogen peroxide is adjusted in the course of the test so as to ensure a molar ratio of hydrogen peroxide to dissolved sodium carbonate in the mother liquors, Term of the upward displacement of the supersaturated aqueous solution, equal to 1.4.

0045The initial solution of sodium carbonate contains 20% sodium carbonate, 7.2% sodium chloride, 800 ppm sodium hexametaphosphate and 520 ppm sodium silicate.

0046The temperature in reactor 1 is maintained substantially between 16.5 ° C and 17.5 ° C and that in container 8 at about 65 ° C.

0047The agglomerates of sodium percarbonate manufactured and accompanied by the mother liquor were withdrawn in time from reactor 1, by 12. These agglomerates are spun off in a centrifuge and then dried in a fluidized bed at 55 ° C. The mother liquor recovered during the spin is returned to the overflow tank 10.

0048After the start-up phase, the linear velocity is equal to 5 m / h in the cylindrical part of the reactor 1 and 40 l / h of the liquid is drawn off at the end of its upward movement.

0049These agglomerates have a bulk density of 0.94 g / cm³.

0050The active oxygen content is 14.6%.

0051The time required to dissolve 90% of 2 g of these agglomerates in one liter of water at 15 ° C. is 70 seconds.

0052The average particle size of these agglomerates is 850 μm and is distributed as follows:<ul><li><160 μm = 1%</li><li>160 - 480 μm = 9%</li><li>480 - 750 μm = 27%</li><li>750 - 1020 μm = 41%</li><li>1020 - 1400 μm = 21%</li><li>> 1400 μm = 1%</li></ul>

0053Weight loss after the attrition test according to ISO 5937 is less than 1%.

0054The <figref idrefs="f0002">Figures 2 and 3</figref> Show the plates of these agglomerates obtained by scanning electron microscopy with a magnification of 12 and 50 respectively.

Example 2

0055The operation is carried out in the same manner as in Example 1 but with a hydrogen peroxide / sodium carbonate molar ratio dissolved in the mother liquors, at the end of the upward displacement of the supersaturated aqueous solution, equal to 1.3.

0056The bulk density of the agglomerates obtained after spin drying is 0.9 g / cm 3 and the average particle size is 830 μm.

0057The time required to dissolve 90% of 2 g of these agglomerates in one liter of water at 15 ° C. is 65 seconds.

0058Weight loss after the ISO 5937 attrition test is less than 1%.

Example 3

0059The procedure is identical to that of Example 1 except that the feed rate of the solid sodium carbonate in the vessel increases to 1000 g / h.

0060The apparent density of the agglomerates obtained after drying is 0.75 g / cm 3 and the average particle size is 610 μm.

0061The time required to dissolve 90% of 2 g of these agglomerates in 1 liter of water at 15 ° C. is 60 seconds.

0062Weight loss after the ISO 5937 attrition test is less than 2%.