EP2142472A1

EP2142472A1 - Process for the preparation of coated sodium percarbonate

Info

Publication number: EP2142472A1
Application number: EP08749855A
Authority: EP
Inventors: Jürgen RABE; Henk Venbrux; Bernd Hoffmann
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 2007-05-02
Filing date: 2008-04-29
Publication date: 2010-01-13
Also published as: CN101687640A; WO2008135462A1

Abstract

A process for the preparation of coated sodium percarbonate containing particles, comprising (a) a manufacturing step of sodium percarbonate containing core particles, comprising the crystallisation of sodium percarbonate from aqueous solution and the separation from aqueous solution, (b) a coating step comprising the application of a base coating on the so obtained core particles with small solid sodium percarbonate containing particles with a mean particle size below or equal to 200 μm, and (c) a drying step of the coated sodium percarbonate containing particles, wherein step (b) and optionally (c) are carried out in a fluid bed reactor.

Description

Process for the preparation of coated sodium percarbonate

The present application claims the benefit of the European application no. 07107378.7 filed on May 2, 2007, herein incorporated by reference. Introduction

The present invention relates to an enhanced process for the preparation of coated sodium percarbonate (PCS) containing particles, the so obtained particles, as well as their use in detergent compositions.

The use of sodium percarbonate (or sodium carbonate peroxyhydrate, 2 NaCO₃ . 3 H₂O₂) as bleaching agent in detergent compositions for household fabric washing or dish washing is well known. Commonly such detergent compositions contain among other components zeolites as builder material, enzymes, bleach activators and/or perfumes.

Different processes are known to produce sodium percarbonate, among them so-called crystallisation processes comprising the crystallisation of sodium percarbonate from aqueous solution and the separation from this aqueous solution, e.g. with salting-out agents, such as sodium chloride, etc. Other processes make use of fluid bed reactors, wherein small seed particles are grown by injecting solutions of sodium carbonate and hydrogen peroxide in the appropriate stoechiometric ratio.

While crystallisation processes need less energy, they generally suffer from the drawback that the resulting particles often contain salting-out agents and that due to the irregular shaped particles having a large surface to volume ratio, these particles are more likely to be prone to attrition and early percarbonate decomposition.

On the other hand, fluid bed processes yield particles with a smooth surface and good attrition behaviour, however the need to introduce the reactants in solution and the subsequent energy intensive evaporation is economically detrimental.

Hence, in an effort to benefit from both techniques to reduce costs and nevertheless obtain PCS particles with good properties, both processes have been combined, e.g. by coating in the fluid bed reactor seed particles obtained by crystallisation from solution. However, as the amount of sodium percarbonate based raw materials for use in the manufacture of detergent, dishwashing and similar household and industrial compositions shipped worldwide represents approximately 500 000 MT per year with increasing tendency, one can easily imagine the economic impact even of a small reduction of costs resulting from an enhanced process. Object of the invention

The object of the present invention is to provide an optimized and more energy efficient process for producing coated sodium percarbonate particles having a high percarbonate content, being highly soluble and having good attrition resistance.

General description of the invention

In order to overcome the abovementioned problems, the present invention provides for a process for the preparation of coated sodium percarbonate containing particles, comprising the following steps: (a) a manufacturing step of sodium percarbonate containing core particles, comprising the crystallisation of sodium percarbonate from aqueous solution and the separation from aqueous solution,

(b) a coating step comprising the application of a base coating on the so obtained core particles with small solid sodium percarbonate containing particles with a mean particle size below or equal to 200 μm embedded in sodium percarbonate optionally comprising one or more additive(s), and

(c) a drying step of the coated sodium percarbonate containing particles, wherein step (b) and optionally (c) are carried out in a fluid bed reactor.

One of the essential characteristics of the present invention resides in the presence in the coating layer surrounding the sodium percarbonate core particles of small sodium percarbonate particles embedded in sodium percarbonate optionally also comprising one or more additives.

The main advantage of the invention is that, due to the introduction of small solid percarbonate particles in the coating step, less water (both from sodium carbonate solution and from the hydrogen peroxide solution) needs to be evaporated in the fluid bed reactor(s), which has a positive impact on the energy requirements and consequently on the costs. A further benefit is that, due to the bed or matrix of PCS, the overall AvOx content is very high.

Furthermore, it has been observed that by introducing small sodium percarbonate particles into the coating layer, the stability of the coated sodium percarbonate particles, when they are stored for a long period before being used, is improved.

In a preferred embodiment, the sodium percarbonate containing core particles from step (a) are subjected to an at least partial preliminary drying step. In this context, an at least partial drying preferably yields particles having from 5 to 20 % by weight, preferably from 10 to 15 % by weight of free water content.

Furthermore, the coating in step (b) preferably comprises one or more agglomeration agent(s) to ensure a good adhesion on the core particles and inside the coating. Those agglomeration agents may be selected from any appropriate agent promoting said adhesion and they are advantageously chosen from water, preferably the water contained in a sodium percarbonate containing or generating solution or suspension or in an aqueous solution or suspension of at least one additive.

In the above process, these sodium percarbonate containing or generating solutions and/or suspensions are preferably chosen from (1) a solution or suspension of sodium carbonate and sodium percarbonate, (2) a solution or suspension of sodium carbonate optionally comprising sodium percarbonate and a solution of hydrogen peroxide or (3) a solution or suspension of sodium carbonate, a solution or suspension of sodium percarbonate and a solution of hydrogen peroxide.

The small sodium percarbonate particles used in the present invention have a mean particle size smaller than 200 μm, in particular smaller than 175 μm, more particularly smaller than 150 μm, values smaller than 125 μm giving good results. Usually, the small sodium percarbonate particles have a mean particle size of at least 1 μm, especially at least 5 μm, most often at least 10 μm and in particular of at least 25 μm. Generally, at least 90 (in particular at least 95, and more particularly at least 99) % by weight of the small sodium percarbonate particles have a diameter below 250 μm (especially below 220 μm, and most preferably below 200 μm). The coated sodium percarbonate particles of the present invention have a mean particle size of at least 300 μm, in particular at least 400 μm, and more particularly at least 500 μm. The mean particle size is at most 1600 μm, especially at most 1400 μm, values of at most 1000 μm being preferred, for instance at most 800 μm. The mean particle size of particles may be measured using a sieve set

(containing at least 6 sieves of known sieve aperture) to obtain several fractions - A -

and weighing each fraction. The mean particle size in μm (MPS) is then calculated according to the formula in which n is the number of sieves (not including the sieve pan), Hi₁ is the weight fraction in % on sieve i and Ic₁ is the sieve aperture in μm of sieve / . The index i increases with increasing sieve aperture. The sieve pan is indicated with the index 0 and has an aperture of ko = 0 μm and mo is the weight retained in the sieve pan after the sieving process. k_n+i equals to 1800 μm and is the maximum size considered for the MPS calculation. A typical sieve set which gives reliable results is defined as follows: n = 6; kβ = 1400 μm; £₅ = 1000 μm; fø = 850 μm; k₃ = 600 μm; k₂ = 425 μm; h = 150 μm.

The coating layer(s) present in the coated sodium percarbonate particles of the present invention represent(s) in general at least 0,1 % by weight of the core particles, in particular at least 0,5 % by weight and most preferably at least 1 % by weight. The coating layer(s) represent(s) in many cases at most 50 % by weight of the core particles, especially at most 35 % by weight, and most often at most 25 % by weight. Amounts of from 0,1 to 50 % by weight give good results.

The coated sodium percarbonate particles of the invention have a good storage or in-detergent stability, and especially long-term storage stability, which can be expressed in two different ways.

According to the first way, it is expressed as heat output at 40 ⁰C measured after storage of 1 g of the product during 12 weeks at 40 ⁰C in a closed ampoule of 3,5 ml. The measurement of heat output by microcalorimetry consists of using the heat flow or heat leakage principle using a LKB2277 Bio Activity Monitor. The heat flow between an ampoule containing the coated sodium percarbonate particles and a temperature controlled water bath is measured and compared to a reference material with a known heat of reaction. This long-term stability is generally less than 10 μW/g, in particular less than 8 μW/g, preferably less than 6 μW/g, and most preferably less than 4 μW/g. According to the second way, the long-term stability is expressed as the

AvOx (or available oxygen content) recovery after storage of 1 g of the product for 8 weeks at 55 ⁰C in a closed ampoule of 3,5 ml. The AvOx recovery corresponds to the difference between the available oxygen content before and after the storage expressed as percentage of the initial available oxygen content. The available oxygen content is measured as explained below. This AvOx recovery is in many cases at least 60 %, especially at least 70 %, values of at least 75 % being very suitable, those of at least 80 % being preferred.

The coated sodium percarbonate particles of the invention have usually a content of available oxygen of at least 12,0 % by weight, in particular at least 13,0 % by weight, contents of at least 13,5 % by weight being particularly satisfactory. The content of available oxygen is generally at most 15,0 % by weight, in particular at most 14,0 %, for instance at most 14,2 %. The content of available oxygen is measured by titration with potassium permanganate after dissolution in sulfuric acid (see ISO standard 1917-1982). The coated sodium percarbonate particles of the present invention usually have a 90 % dissolution time of at least 0,1 min, in particular at least 0,5 min. Generally, the 90 % dissolution time is at most 3 min, especially at most 2,5 min. The 90 % dissolution time is the time taken for conductivity to achieve 90 % of its final value after addition of the coated sodium percarbonate particles to water at 15 +/-1 ⁰C and 2 g/1 concentration. The method used is adapted from ISO

3123-1976 for industrial perborates, the only differences being the stirrer height of 10 mm from the beaker bottom and a 2 liter beaker (internal diameter 120 mm).

The coated sodium percarbonate particles of the present invention usually have a bulk density of at least 0,8 g/cm³, in particular at least 0,9 g/cm³. It is generally at most 1,2 g/cm³, especially at most 1,1 g/cm³. The bulk density is measured by recording the mass of a sample in a stainless steel cylinder of internal height and diameter 86,1 mm, after running the sample out of a funnel (upper internal diameter 108 mm, lower internal diameter 40 mm, height 130 mm) placed 50 mm directly above the cylinder.

The coated sodium percarbonate particles of the invention usually have an attrition measured according to the ISO standard method 5937-1980 of at most 8 %, in particular at most 5 %, especially at most 3 %. The attrition is in most cases at least 0,05 %. In a further embodiment, the above process further comprises one or more additional coating steps between steps (b) and (c), one, more or all of these additional coating step(s) being optionally associated with (e.g. preceded by or concomitant to) an at least partial drying step, wherein each additional coating step comprises the coating of the particles from the previous step according to step (b) or with one or more additives and optionally one or more sodium percarbonate containing or generating solutions and/or suspensions, if desirable combined with small solid sodium percarbonate containing particles with a mean particle size below or equal to 200 μm, the composition of each coating being different from that of its adjacent coating(s).

Preferably, one or more, more preferably all of the additional coating step(s) and if applicable one or more, more preferably all of their optionally associated drying step(s) are carried out in fluid bed reactor(s).

The additive(s) used in the additional coatings or optionally used for the base coating is/are preferably chosen from organic or inorganic stabilizers, builders, alkaline sources, fillers, flowability enhancers and/or glass corrosion protectors, such as alkali metal or alkaline-earth metal sulphates, bicarbonates, carbonates, citrates, phosphates, borates, silicates and/or chlorides, as well as their hydrates, polycarboxylate, polyphosphonate or polyhydroxyacrylate salts, as such or in acid form, for example polyaminocarboxylates like EDTA or DTPA, or polyaminomethylene-phosphonates like EDTMPA, CDTMPA or DTPMPA, or hydroalkylenephosphonates like hydroxyethylidenediphosphonate), or from mixtures of the above.

It is to be understood that the process may also comprise at least one sieving step, e.g. after step (c) or at any appropriate point in the process, e.g. to collect undesirably fine size fractions of particles, and to recycle them at any appropriate step, preferably the collected particles have a mean particle size below or equal to 200 μm and they are recycled in coating step (b). These small particles may also be obtained from larger particles, such as by milling.

The small solid sodium percarbonate containing particles used in coating step (b) or in further coating step(s) may also be chosen from recycled particles carried out with the fluidising gas, optionally after milling.

As a further aspect, the invention pertains to coated sodium percarbonate particles obtained by a process as described above. Hence, such particles comprise a sodium percarbonate containing core, obtained by the crystallisation of sodium percarbonate from aqueous solution and the separation from aqueous solution, and on the core a sodium percarbonate containing base coating comprising small solid sodium percarbonate containing particles with a mean particle size below or equal to 200 μm embedded in sodium percarbonate optionally comprising one or more additive(s).

These particles may further comprise on said base coating one or more additional coating(s) containing sodium percarbonate and/or one or more additive(s), the composition of each additional coating being different from that of its adjacent coating(s).

Further characteristics of the coated sodium percarbonate particles are as already described above.

A still further aspect of the invention pertains to the use of the coated sodium percarbonate particles as described above as bleaching agent in detergent compositions.

Hence, a still further aspect of the invention is concerned with detergent compositions containing such coated sodium percarbonate particles.

Claims

C L A I M S

1. A process for the preparation of coated sodium percarbonate containing particles, comprising the following steps:

(a) a manufacturing step of sodium percarbonate containing core particles, comprising the crystallisation of sodium percarbonate from aqueous solution and the separation from aqueous solution,

(c) a drying step of the coated sodium percarbonate containing particles,

wherein step (b) and optionally (c) are carried out in a fluid bed reactor.

2. The process according to claim 1, wherein the sodium percarbonate containing core particles from step (a) are optionally subjected to a preliminary drying step and wherein the coating in step (b) also comprises one or more agglomeration agent(s).

3. The process according to claim 2, wherein the agglomeration agents are chosen from water, a sodium percarbonate containing or generating solution or suspension or an aqueous solution or suspension of at least one additive.

4. The process according to claim 3, wherein the sodium percarbonate containing or generating solutions and/or suspensions are chosen from (1) a solution or suspension of sodium carbonate and sodium percarbonate, (2) a solution or suspension of sodium carbonate optionally comprising sodium percarbonate and a solution of hydrogen peroxide or (3) a solution or suspension of sodium carbonate, a solution or suspension of sodium percarbonate and a solution of hydrogen peroxide.

5. The process according to any of claims 1 to 4, further comprising one or more additional coating steps between steps (b) and (c), one, more or all of these additional coating step(s) being optionally associated with an at least partial drying step, wherein each additional coating step comprises the coating of the particles from the previous step according to step (b) or with one or more additives and optionally one or more sodium percarbonate containing or generating solutions and/or suspensions.

6. The process according to claim 5, wherein one, more or all of the additional coating step(s) and if applicable one, more or all of their optionally associated drying step(s) are carried out in fluid bed reactor(s).

7. The process according to any one of claims 3 to 5, wherein the additive(s) is/are chosen from organic or inorganic stabilizers, builders, alkaline sources, fillers, flowability enhancers and/or glass corrosion protectors, such as alkali metal or alkaline-earth metal sulphates, bicarbonates, carbonates, citrates, phosphates, borates, silicates and/or chlorides, as well as their hydrates, polycarboxylate, polyphosphonate or polyhydroxyacrylate salts, as such or in acid form, or from mixtures of the above.

8. The process according to any of the preceding claims wherein the coated particles have a mean particle size from 300 to 1600 μm.

9. The process according to any of the preceding claims, further comprising at least one sieving step wherein the particles with a mean particle size below or equal to 200 μm are recycled in coating step (b).

10. The process according to any of the preceding claims, wherein small solid sodium percarbonate containing particles used in coating step (b) or in further coating step(s) are chosen from recycled particles carried out with the fluidising gas and recycled particles from the sieving step(s), optionally after milling.

11. Coated sodium percarbonate particles obtained by a process according to claims 1 to 10.

12. The particles according to claim 11, comprising a sodium percarbonate containing core, obtained by the crystallisation of sodium percarbonate from aqueous solution and the separation from aqueous solution, and on the core a sodium percarbonate containing base coating comprising small solid sodium percarbonate containing particles with a mean particle size below or equal to 200 μm embedded in sodium percarbonate optionally comprising one or more additive(s).

13. The particles according to claim 12, comprising one or more additional coating(s) containing sodium percarbonate and/or one or more additive(s).

14. The particles according to claim 12 or 13, wherein the additive(s) is/are chosen from organic or inorganic stabilizers, builders, alkaline sources, fillers, flowability enhancers and/or glass corrosion protectors, such as alkali metal or alkaline-earth metal sulphates, bicarbonates, carbonates, citrates, phosphates, borates, silicates and/or chlorides, as well as their hydrates, polycarboxylate, polyphosphonate or polyhydroxyacrylate salts, as such or in acid form, or from mixtures of the above.

15. The particles according to any of claims 12 to 14, having a mean particle size from 300 to 1600 μm.

16. Use of the coated sodium percarbonate particles according to any one of claims 11 to 15 as bleaching agent in detergent compositions.

17. Detergent compositions containing the sodium percarbonate particles of any one of claims 11 to 15.