AU747168B2

AU747168B2 - Stable supersaturated sodium perborate solution and its use for making stabilised sodium percarbonate particles

Info

Publication number: AU747168B2
Application number: AU89881/98A
Authority: AU
Inventors: Jean-Michel Bossoutrot; Francois Garcia
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 1997-09-05
Filing date: 1998-08-05
Publication date: 2002-05-09
Anticipated expiration: 2018-08-05
Also published as: WO1999012848A1; KR100527255B1; FR2768140A1; EP1023239A1; AU8988198A; CN1277593A; FR2768140B1; JP2001515831A; KR20010023711A; CN1134374C

Description

WO 99V/128488 1 PCT/FR98/01741 STABLE SUPERSATURATED SOLUTION OF SODIUM PERBORATE AND ITS APPLICATION IN THE MANUFACTURE OF STABILIZED PARTICLES OF SODIUM PERCARBONATE The present invention relates to a stable supersaturated aqueous solution of sodium perborate and to its use in stabilizing sodium percarbonate particles.

Sodium percarbonate (2Na 2 CO3'3H 2 0 2 particles are used as active oxygen compounds in washing powders, bleaching agents and detergents. Because of the inadequate stability of these particles during storage in warm and humid surroundings and in the presence of various components of washing powders and detergents, the sodium percarbonate particles have to be stabilized. The method commonly employed to stabilize sodium percarbonate particles consists in coating them with a covering of components with a stabilizing action.

The use of a covering of components with a stabilizing action based on sodium perborate has formed the subject of many studies. Thus, Application WO 94 20413 discloses a process for the stabilization of sodium percarbonate particles by spraying these particles with an aqueous solution or suspension comprising sodium perborate hydrate in a proportion of to 450 g/l. However, this process exhibits disadvantages for sodium perborate hydrate contents greater than the value of its solubility in water. This is because uniform and effective coverage of the percarbonate particles cannot be achieved with a sodium perborate suspension.

Furthermore, Application FR 2,419,252 discloses a process for the manufacture of stabilized particles of sodium percarbonate by spraying these particles, in several stages, with a dilute aqueous sodium perborate solution and with a sodium silicate solution. This process exhibits the disadvantage of requiring several intermediate spraying and drying stages. Furthermore, 2 the particles, thus stabilized, of sodium percarbonate have a solubility which is not very satisfactory and result in an undesirable proportion of insoluble components.

By virtue of the discovery of a stable supersaturated solution of sodium perborate, the Applicant Company has now found a process for the manufacture of stabilized particles of sodium percarbonate which do not exhibit the disadvantages stated above.

The stable supersaturated aqueous solution of sodium perborate according to the invention, prepared from sodium metaborate, hydrogen peroxide and sodium silicate, comprises from 4 to 49% by weight of sodium perborate tetrahydrate, preferably from 9 to 49% by weight, and sodiumsilicate in an amount such that the sodium metaborate involved/sodium silicate ratio by mass is between 0.6 and 7.3 and preferably between 0.9 and 3.

Such a solution can be obtained by mixing sodium metaborate, sodium silicate and aqueous hydrogen peroxide solution in a sodium metaborate: hydrogen peroxide molar ratio of approximately 1:1.

Such a solution is preferably prepared by bringing an aqueous sodium metaborate solution comprising sodium silicate into contact with an aqueous hydrogen peroxide solution. The aqueous solutions advantageously used are those having a molar concentration of hydrogen peroxide similar to that of sodium metaborate.

The various components can be mixed in a reactor or a mixer (stirred batch reactor, static mixer) This supersaturated aqueous solution is stable within wide temperature limits, generally of between and 500C and preferably of between 20 and The solubility of a solid compound in a given solvent represents the maximum amount (by mass or by 3 moles) of this solid which it is possible to dissolve in a certain volume of solvent at a given temperature.

A solution is said to be supersaturated when its concentration of solute is greater than the value of the concentration corresponding to the solubility at the same temperature. Such a solution will change with the release of the excess solute in solid form until the concentration of solute reaches the value of the solubility. Crystallization or precipitation then takes place.

Supersaturation represents the divergence from solubility and can be defined as being the ratio of the concentration of the solute to the concentration at solubility.

The supersaturated solution can exhibit a delay time for release of the excess solute in solid form.

The term for this is latency period or induction period.

The Applicant Company has therefore discovered that the presence of sodium silicate, in proportions indicated hereinabove, makes it possible to greatly increase the latency period of a supersaturated aqueous solution of sodium perborate and thus to stabilize the supersaturated solution of sodium perborate and to facilitate its processing in the various applications, such as in particular in the process for the manufacture of stabilized particles of sodium percarbonate.

Another subject-matter of the present invention is a process for the manufacture of stabilized particles of sodium percarbonate which are composed of a core made of sodium percarbonate and of a covering comprising sodium perborate tetrahydrate and sodium silicate. This process is characterized in that the wet particles of sodium percarbonate, directly after they have been manufactured or optionally followed by a drying stage, are sprayed with stable supersaturated -4 aqueous solutions of sodium perborate described hereinabove.

The stable supersaturated aqueous solutions of sodium perborate according to the invention which are very particularly suitable for the process for the manufacture of stabilized particles of sodium percarbonate have a latency period which is greater than the time needed to carry out the coating. A latency period of greater than 15 minutes is preferred.

A latency period of greater than 20 minutes is particularly preferred.

The use of a supersaturated solution of sodium perborate makes it possible to uniformly wet the sodium percarbonate particles, whereas a suspension will have more difficulties in spreading at the surface of these particles. In addition, the highly supersaturated solutions exhibit the advantage of introducing a limited amount of water during the coating, thus avoiding the problems of local solidification or of caking, dissolution or recrystallization.

Furthermore, this small amount of water introduced by highly supersaturated solutions of sodium perborate makes it possible to offset the water possibly present in the wet particles of sodium percarbonate obtained directly after manufacture. Thus, sodium percarbonate particles having a moisture content of between 0 and 10% by weight, preferably of between 0 and 7% by weight, can be stabilized.

The amount of water introduced per 100 g of dry particles of sodium percarbonate is generally between 3 and 15 g and preferably between 3 and 12 g.

The sodium percarbonate particles to be stabilized are prepared from sodium carbonate and an aqueous hydrogen peroxide solution, either by the dry route or by the wet route. The latter is advantageously chosen.

The coating of the sodium percarbonate particles by supersaturated solutions of sodium 5 perborate can be carried out in a mechanical or centrifugal mixer or a filtration device. The amount of supersaturated solution of sodium perborate to be employed in stabilizing the sodium percarbonate particles can vary within wide limits. It is generally between 25 and 600 g per 1 kg of dried particles of sodium percarbonate and preferably between 50 and 300 g.

The particles, thus stabilized, of sodium percarbonate are subsequently dried in a fluidized bed dryer at a temperature of between 30 and 80*C and preferably of between 50 and 700C.

The scope of the invention will not be exceeded when the drying is carried out simultaneously with the spraying stage.

The stability of the particles thus obtained of sodium percarbonate is evaluated by determining the loss in active oxygen of a sample placed for one week in an oven at 300C with a relative humidity of The rate of dissolution of these particles, thus stabilized, is determined by the time needed to obtain the dissolution of 90% of 2 g of sodium percarbonate particles which are introduced into one litre of water at 150C.

EXPERIMENTAL PART In what follows and unless otherwise indicated, the aqueous hydrogen peroxide solution comprises 70% by weight of hydrogen peroxide.

The sodium silicate used is of the 7 N 34 type sold by Rh6ne-Poulenc and has a solids content of 34% (26.2% SiO 2 and 7.8% Na 2 0 by weight).

An aqueous solution A is prepared from a 33% sodium metaborate solution, sodium silicate 7N34 and water. An aqueous solution B is also prepared which has a molar concentration of hydrogen peroxide similar to the concentration of sodium metaborate in the solution A. The aqueous solution A and then the aqueous solution 6 B are introduced into a stirred 250 cm 3 reactor which is thermostatically controlled at 23 0 C. The end of the introduction of the aqueous solution B is taken as time zero and the time needed for the appearance of the first crystals of sodium perborate tetrahydrate (or the latency period) is measured visually.

[See Table 1].

7 TABLE 1 SOLUTION A SOLUTION B Solid Metaborate Silicate Water to 70% H 2 0 2 Water to be of sodium Latency metaborate sol. solution be added added perborate period comprising 7N34 tetrahydrate (min) 33% of NaB02 (g) 3.89 11.79 0 42.7 2.87 42.7 9 4 3.89 11.79 6.17 39.6 2.87 39.6 9 9 3.89 11.79 18.51 33.4 2.87 33.4 9 10.4 31.52 0 30.4 7.6 30.4 24 10.4 31.52 8.25 26.3 7.6 26.3 24 7 10.4 31.52 16.5 22.2 7.6 22.2 24 18.5 56.06 0 15.2 13.57 15.2 43 18 18.5 56.06 14.65 7.9 13.57 7.9 43 28 18.5 56.06 29.34 0.5 13.57 0.5 43 8 Quantitative determination of the active oxygen Reagents used: N/10 Potassium permanganate

(K)

Sulphuric acid (NFT Standard 73.703), prepared by dissolving 50 g of aluminium sulphate nonahydrate, 5 g of bismuth nitrate pentahydrate and 5 g of manganese sulphate monohydrate in 1 litre of 5N sulphuric acid.

Procedure: 2 g of product are precisely weighed in a volumetric flask and then 100 ml of demineralized water are added thereto, followed by 25 ml of sulphuric acid (NFT Standard 73.703). The solution is subsequently titrated with potassium permanganate until a permanent pink colouring is obtained.

If the test sample weighs p grams and the volume of permanganate run in is n ml, the percentage of active oxygen is given by the following formula: of active oxygen 0.8 x K x n p as K 0.1N, the of active oxygen 0.08n p The loss of active oxygen is defined as being the difference between the initial active oxygen, with regard to 2 g, and the final active oxygen, with regard to the final weight, expressed as percentage of the initial content.

Example 900 g of dried particles of sodium percarbonate having the following characteristics:

H

2 0 2 by weight 31 Na 2

CO

3 by weight 65.4 Apparent bulk density 0.89 g/cm 3 Rate of dissolution 64 s 9 Mean diameter of the particles 800 pm are introduced into a 5 litre stainless steel mechanical mixer (company Controlab, EN Standard 196- 1).

The supersaturated aqueous solution of sodium perborate prepared from 10.6 g of sodium metaborate, 7.8 g of the aqueous hydrogen peroxide solution, 16.9 g of sodium silicate 7 N 34 and 67.5 g of demineralized water is then introduced over 16 minutes while stirring at approximately 140 revolutions/min. The mixture is subsequently left stirring for an additional two minutes. The particles, thus coated, of sodium percarbonate are subsequently dried in a fluidized bed at approximately 55 0 C. On conclusion of the drying stage, the amount of sodium percarbonate particles which is recovered is 922 g and the particles exhibit the following characteristics:

H

2 0 2 by weight 31.1 Na2C0 3 by weight 64 Apparent bulk density 0.95 g/cm 3 Rate of dissolution 76 s Mean diameter 750 pm Loss of active oxygen 0.6% Example 11 (not in accordance with the invention The procedure is as described in Example except that use is made of a suspension of sodium perborate tetrahydrate comprising 21.62 g of fine particles of sodium perborate with a diameter of less than or equal to 50 pm, 1.32 g of sodium metaborate, 16.9 g of sodium silicate 7 N 34, 1.14 g of the aqueous hydrogen peroxide solution and 61.94 g of demineralized water. The characteristics of the particles, thus stabilized, of sodium percarbonate are as follows:

H

2 0 2 by weight 31.1 Na 2

CO

3 by weight 63.9 10 0 0 0 .0* 0 *r 00 0 .0.0 0000 000.

.000 00 0

S

Apparent bulk density 0.93 g/cm 3 Rate of dissolution 80 s Loss of active oxygen 1% Example 12 The procedure is as described in Example except that the time for introduction of the supersaturated solution of sodium perborate is 43 minutes and that the mixture is subsequently left stirring for an additional 3 minutes.

Unstabilized Stabilized sodium sodium percarbonate percarbonate H202 by weight 31.2 31.2 Na 2

CO

3 by weight 65.8 64.5 Apparent bulk density (g/cm 3 0.87 1.07 Rate of dissolution 58 76 Mean diameter (pm) 840 740 Example 13 The procedure is as described in Example except that the sodium percarbonate particles to be coated have a moisture content of 5% and that the supersaturated solution of sodium perborate comprises 43% by weight of sodium perborate and sodium silicate in an amount such that the sodium metaborate involved/sodium silicate ratio by mass is identical to that in Example The particles, thus stabilized, of sodium percarbonate have characteristics identical to those obtained according to Example Throughout the description and claims of this specification, the word "comprise" and variations 'of the word, such as "comprising" and "comprises", is not intended to exclude other additives of components or integers or steps.

10A The above discussion of documents, act, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application.

e*

Claims

1. Supersaturated solution of sodium perborate prepared from sodium metaborate, hydrogen peroxide and sodium silicate, comprising from 4 to 49% by weight of sodium perborate tetrahydrate and sodium silicate in an amount such that the sodium metaborate involved/sodium silicate ratio by mass is between 0.6 and 7.3.

2. Supersaturated solution according to claim 1, wherein said solution comprises from 9 to 49% by weight of sodium perborate tetrahydrate and sodium silicate in an amount such that the sodium metaborate involved/sodium silicate ratio by mass is between 0.6 and 7.3.

3. Supersaturated solution according to claim 1 or 2, wherein said solution comprises from 9 to 49% by weight of sodium perborate tetrahydrate and sodium silicate in an S* amount such that the sodium metaborate involved/sodium silicate ratio by mass is between 0.9 and 3.

4. Supersaturated solution according to any one of claims 1 to 3, wherein said solution comprises from 4 to 49% by weight of sodium perborate tetrahydrate and sodium silicate in an amount such that the sodium metaborate involved/sodium silicate ratio by mass us between 0.9 and 3. Process for the manufacture of stabilized particles of sodium percarbonate which are composed of a core made of sodium percarbonate and of a covering comprising sodium perborate tetrahydrate and sodium silicate, wherein the wet particles of sodium percarbonate, directly after they have been manufactured, or those obtained on conclusion of the drying stage are sprayed with the stable supersaturated solution of sodium perborate according to any one of Claims 1 to 4. 11A

6. A supersaturated solution according to claim 1 substantially as hereinbefore described with reference to any one of the Examples.

7. A process according to claim 5 substantially as hereinbefore described. DATED: 19 April, 2000 PHILLIPS ORMONDE FITZPATRICK Attorneys for: SOLVAY (SOCIETE ANONYME)