Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
1997-03-18
2002-01-22
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C423S266000, C023S30200R
Reexamination Certificate
active
06340452
ABSTRACT:
1. BACKGROUND OF THE INVENTION
1.1 Technical Field
The present invention relates to a novel sodium percarbonate, of overall formula Na
2
CO
3
·1.5 H
2
O
2
, and to a process for manufacturing it. Sodium percarbonate is also known by the names sodium carbonate sesqui(peroxyhydrate), sodium carbonate peroxyhydrate, or sodium carbonate peroxide.
1.2 Description of The Related Art
The sodium percarbonate known to date is either:
a monocrystal in the form of a hexagonal prism with a density of between 0.9 and 1 g/cm
3
(FR 2318112), or
a monocrystal in the form of a regular rhombohedral (FR 2355774), or
a hollow granule with an apparent density of about 0.4 g/cm
3
and with an average diameter of about 480 &mgr;m (FR 2486056), or
in the form of a compact grain with an average particle size of about 450 &mgr;m (BE 859155).
A sodium percarbonate of very high quality, consisting of agglomerated of small crystals, has now been found.
Moreover, it is known to manufacture sodium percarbonate by reaction of a solution or suspension of sodium carbonate with aqueous hydrogen peroxide solution. The addition of inert salts such as sodium chloride to lower the solubility of the sodium percarbonate is also known.
Processes are known for the batchwise manufacture of sodium percarbonate (See, for example, BE 859155 and FR 2368438).
Processes are also known for the continuous manufacture of sodium percarbonate, working either under vacuum (FR 2318112) or with two reactors in series (EP 496430) and leading, respectively, to monocrystals and to crystals of sodium percarbonate less than 75 &mgr;m in size.
A process for the continuous manufacture of sodium percarbonate has now been discovered based on the generation and agglomeration of small crystals of the said percarbonate from an aqueous supersaturated solution of sodium percarbonate.
2. DESCRIPTION OF THE INVENTION
The sodium percarbonate of the present invention consists of agglomerates of small crystals of sodium percarbonate. The size of the small crystals is generally between about 1 &mgr;m and about 100 &mgr;m and preferably between about 5 &mgr;m and about 20 &mgr;m. The material of the present invention is characterized by very good resistance to attrition, preferably with a weight loss of fines of less than 1% according to the ISO standard test 5937 (method of friability in a fluid bed).
The sodium percarbonate in agglomerate form according to the invention has an apparent density of greater than about 0.6 g/cm
3
and preferably between about 0.75 g/cm
3
and about 1.1 g/cm
3
.
The sodium percarbonate agglomerates of the invention have the advantage of dissolving rapidly. Thus, the time required to obtain 90% dissolution of 2 g of sodium percarbonate agglomerates introduced into one liter of water is generally less than or equal to 90 seconds at 15° C. and preferably between 40 and 70 seconds.
The sodium percarbonate in accordance with the invention is formed of agglomerates whose average particle size varies with the conditions chosen to manufacture them.
These conditions, described hereunder, make it possible to obtain agglomerates whose particle size may be between wide limits from about 160 &mgr;m to about 1400 &mgr;m with a particularly narrow particle size distribution. For example, in the case of agglomerates with an average particle size equal to about 765 &mgr;m, generally at least 85% are between about 480 &mgr;m and about 1400 &mgr;m in size.
Advantageously, the average particle size of these agglomerates is greater than 600 &mgr;m and preferably greater than 700 &mgr;m.
The content of fines smaller than 160 &mgr;m in size present with the agglomerates as obtained from the process which is suitable for manufacturing them is less than 2% by weight, usually even less than 1% by weight.
The active oxygen content of these sodium percarbonate agglomerates is generally greater than 14% and is defined as being the percentage by mass of the amount of oxygen available relative to the sodium percarbonate agglomerates.
A second subject of the present invention is 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.
This process is characterized in that it comprises a reactor within which is a bed of small crystals and/or agglomerates of sodium percarbonate. A supersaturated solution of sodium percarbonate is formed by placing a suspension or solution of sodium carbonate in contact with an aqueous hydrogen peroxide solution. The bed is maintained in suspension by an ascending stream of the aqueous supersaturated solution of the said percarbonate. The supersaturated state of the aqueous solution decreases as it moves in a continuously ascending motion at a speed such that a desired particle-size classing of agglomerates is ensured.
In general, the process for producing sodium percarbonate agglomerates according to the present invention comprises the following steps:
a) forming an aqueous supersaturated solution of sodium percarbonate in a reactor by placing a suspension or a solution of sodium carbonate in contact with an aqueous hydrogen peroxide solution,
b) charging the reactor with a bed of sodium percarbonate, wherein the sodium percarbonate is present in the form of small crystals, agglomerates, or mixtures thereof, and
c) maintaining the bed of sodium percarbonate in suspension by contacting the bed with an ascending stream of the aqueous supersaturated solution of sodium percarbonate, wherein the solution forms a stream which ascends in a path and the linear ascending speed of the stream is between 2 m/h and 20 m/h.
The sodium percarbonate agglomerates thusly manufactured are removed from the suspension in the lower part of the bed.
At the end of its ascending movement, the aqueous solution is supersaturated with sodium percarbonate and has a sodium percarbonate concentration generally between the value corresponding to the solubility of sodium percarbonate in the same medium at the same temperature and about 1.6 times that value. At the end of its ascending motion, the aqueous solution preferably has a sodium percarbonate concentration of between about 1 and about 1.4 times the value corresponding to the solubility of sodium percarbonate in the same medium at the same temperature. The amount of solid material present at this level in the reactor 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.
In order to form the sodium percarbonate needed to ensure the supersaturated state of the aqueous supersaturated solution of sodium percarbonate, amounts of hydrogen peroxide and of sodium carbonate are used such that the molar ratio of hydrogen peroxide to sodium carbonate dissolved in the mother liquor, at the end of the ascending motion of the aqueous supersaturated solution, is greater than 1 and preferably between about 1.2 and about 1.6.
The supplying of sodium percarbonate as an aqueous solution to ensure this supersaturated state may be performed by formation of the said percarbonate in the aqueous supersaturated solution of percarbonate itself or outside this solution. The formation of sodium percarbonate in the aqueous supersaturated solution of sodium percarbonate is particularly preferred and it is performed by continuous introduction of aqueous hydrogen peroxide solution and of a suspension or solution of sodium carbonate, optionally containing sodium percarbonate in solution or suspension. Preferably, the aqueous hydrogen peroxide solution is introduced into the ascending path of the aqueous supersaturated solution of sodium percarbonate at a level very close to the zone of introduction of the sodium carbonate suspension or solution. The introductions of the aqueous hydrogen peroxide solution and of the sodium carbonate suspension or solution may also take place at several levels.
Preferably, the sodium carbonate suspension or solution is introduced into the zone between the level located at the upper limit of r
Bossoutrot Jean-Michel
Garcia François
Hendrickson Stuart L.
Solvay
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