Preparing a crystalline, stable sodium percarbonate

Details Preparing a crystalline, stable sodium percarbonate Preparing a crystalline, stable sodium percarbonate

1995-04-25

1997-02-18

Langel, Wayne

Chemistry of inorganic compounds

With additive

For stabilizing crystal size or shape

423274, 4234152, C01B 3124

Patent

active

056039115

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a process for preparing a crystalline, stable sodium percarbonate by crystallizing an aqueous solution of starting materials.
Sodium percarbonate is a water-soluble, crystalline peroxygen compound with the molecular formula .sub.2 Na.sub.2 CO.sub.3. 3H.sub.2 O.sub.2. It has a theoretical active oxygen concentration (AO) of 15.28% by weight. Sodium percarbonate is relatively well water-soluble, sodium carbonate and hydrogen peroxide being released in the solution. Owing to this characteristic sodium percarbonate has, in fact, a widespread application as a bleaching agent. It is particularly suitable for bleaching textiles and for removing coloured stains from textiles in industry and households.
Compared to another common bleaching agent, sodium perborate, sodium percarbonate is considerably faster dissolved in cold water and does not require any special bleaching activator, and is thus also energy-saving. Moreover, compared to perborates and chlorous bleaching agents, sodium percarbonate does not generate any polluting residues.
The stocking of sodium percarbonate involves certain problems. In a cooled, dry storage it is conserved as such for long periods of time. If, however during storage, the relative humidity of air is greater than the critical relative humidity of the product sodium percarbonate will absorb water from the surrounding air. This process is enhanced by a temperature raise and causes a reduction of the AO concentration as a function of time. Sodium percarbonate will be dissolved in the absorbed water into a saturated solution consisting of sodium carbonate and hydrogen peroxide. Sodium carbonate increases the basicity of the solution, which, again, accelerates the natural decomposition of hydrogen peroxide in the solution. Moreover, due to the preparation process and the raw materials, impurities may occur in the product, which accelerate the decomposition of hydrogen peroxide. Such impurities with a catalytic activity are transition metal ions, among others.
In fact, when preparing sodium percarbonate with crystallization methods, it is commonly known to carry out a preliminary purification of sodium carbonate by adding water-soluble Mg salt to its solution, the Mg salt precipitating magnesium carbonate from the solution and metal salt impurities being coprecipitated. In addition to this, the product may be stabilized by adding agents complexing the metal salts concerned and by precipitating finely-divided magnesium silicate in the product, which will protect the hydrogen peroxide in the product against heterogenic decomposition by catalysis of the surfaces of solid impurities.
Because of the catalytic decomposition mechanism, the water absorption tendency of the product can be reduced in the preparation step, since thus the amount of solution phase formed by absorbed water and sodium percarbonate in the finished product, in which dissolved hydrogen peroxide is apt to be decomposed, will be small.
Certain requirements are posed on the physical properties of the product crystals depending on the way the product is used. Thus, the crystals should not generate dust, and also, their size and the bulk density of the product formed by them should be appropriate to be admixed with other detergent components and also adequate for normal packing techniques. These additional requirements have resulted in the practical need to use additives controlling crystal growth in the preparation step, called "crystal modifiers", in order to obtain the desired physical properties of the crystals.
Several methods using crystal modifiers for preparing sodium percarbonate to be used as a bleaching agent are previously known. One way of increasing the stability of sodium percarbonate is to use aminopolycarboxyl acid and/or .alpha.-hydroxyacrylic acid as modifiers in the crystallization, as described in the patent specifications JP 63 147 804, JP 63 215 502 and JP 1153 510 (Nippon Peroxide).
DE 2 364 634 (Degussa) discloses a process for preparing sodium percarbonate, in which polycarboxyl

REFERENCES:
patent: 3883640 (1975-05-01), Smart
patent: 4120812 (1978-10-01), Lutz
patent: 4329244 (1982-05-01), Brichard et al.

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