GB1581465A

GB1581465A - Sodium percarbonate

Info

Publication number: GB1581465A
Application number: GB3149477A
Authority: GB
Original assignee: Interox SA
Current assignee: Solvay Chimie SA
Priority date: 1976-07-27
Filing date: 1977-07-27
Publication date: 1980-12-17
Also published as: LU75466A1; DE2733935C2; FR2359789B1; DE2733935A1; BE857017A; FR2359789A1; IT1085593B

Description

(54) SODIUM PERCARBONATE (71) We, INTEROX, a Societe Anonyme organized under the laws of Belgiurn, of Rue du Prince Albert, 33, B-i 050 Brussels, Belgium, do hereby declare the invention, for which we pray that a patent may be granted to us, tand the method by which it is to be performed, to be particularly described in and by the following staternent:- The present invention relates to sodium percarbonate granules, a process for the production of such granules, and their use in washing powders.

Hitherto, many processes have been proposed for the production of sodium percarbonate granules, but the majority of such processes produce granules which have an undesirably low resistance to attrition, or have relatively poor stability when stored either by themselves, or mixed with other constituents of washing powders.

One interesting process for the production of sodium percabonate granules enables certain of the onatntioned difficulties to be mitigated. The process comprises the steps of intro- ducing a solution of hydrogen peroxide and a solution of sodium carbonate into a fluidized bed containing seeds of smaller size than the granules to be obtained (French Patent No.

2,076,430). The granular product of this process in general has a satisfactory resistance to abrasion, but, in practice, operation of the process can present some difficulties. In fact, if each solution is sprayed into the fluidised bed using a separate atomiser, it is difficult to produce a sufficiently intimate mixture of the two solutions within the fluidised bed to give granules having the desired homogeneity. However, if the two solutions are introduced into the fluidised bed simultaneously by the same atomiser, there is a substantially inoreased tendency for premature crystallisation to take place in the atomiser, leading to blockages of it, which therefore causes frequent stoppages of the plant.

In order to circumvent the disadvantages of introducing solutions directly into the fluidised bed, Solvay et Cie have proposed (British Patent No. 1,403,449) a process in which the seeds are impregnated with the two sol btiens .in a separate stage, the impregnated seeds are then passed into a fluidised bed dreer, and all particles which are smaller than the required particle size are collected at the outlet from the fluidised bed and recycled to the impregnation stage. Although this process produces homogeneous granules, it suffers from the disadvantage that a very large proportion of the granules is recycled and only a small proportion of the granules retained as product, so that the productivity of the apparatus is thus lower than desired.

Moreover, the sodium percarbonate granules obtained by the previously disclosed processes are not sufficiently stable if they are contaminated with certain metal impurities, such as iron.

It !is an object of the present invention to provide a process which ameliorates to at least some extent, one or more of the difficulties described hereinbefore in producing abrasion resistant granules of sodium percarbonate which are storage stable, in at least some embodiments.

According to the present invention there is provided a process for the production of sodium percarbonate granules comprising the steps of:- 1. impregnating seeds, as defined herein with an aqueous phase or phases containing sodium percarbonate or its precursors, 2. evaporating water from the impregnated seeds in a fluidised bed dryer in the presence of at least one condensed phosphate as herein defined.

It will be recognised that when water is evaporated from the impregnated seeds, sodium percarbonate liS crystallised in or on the seeds, so that the average particle size of the seeds before impregnation is smaller than that of the sodium percarbonate granules produced.

Herein, by the term "seed" is meant a particle of a material compatible with sodium percarbonate and having dimensions smaller than those of the product and by the term "condensed phosphate" is meant an alkali metal or ammonium compound containing at least one chain of phosphorous-oxygen-phos- phorous bonds. The condensed phosphate can be a pyrophosphate, a polyphosphate or a metaphosphate. Although any alkali metal, e.g.

potassium, or ammonium condensed phosphate can be used, the sodium salts are preferred.

Preferred condensed phosphate include tetrasodium pyrophosphate, disodium pyrophosphate, pentasodium tripolyphosphate, sodium trimetaphosphate and sodium tetrametaphosphate. Particularly preferred condensed phosphates are metaphosphates, either linear or cyclic, having an atomic ratio of phosphorus to alkali metal of 1:1, especially hexametaphos- prates and advantageously sodium hexametaphosphate.

The condensed phosphate can be introduced into the dryer by various routes, which can be divided into methods in which the condensed phosphate is introduced directly into the bed, and methods in which the seeds are first impregnated with the condensed phosphate and then introduced into the fluid bed. Usually the condensed phosphate iis used in the form d an aqueous solution. The condensed phosphate can be introduced into the bed either by itself, generally by spraying an aqueous solution, Of dissolved dn the aqueous phase or phases con tailing sodium percarbonate or its precursors.

Independent upon whether the condensed phosphate is introduced by itself or together with sodium percarbonate Of a precursor thereof, the condensed phosphate may be introduced directly into the fluidised bed, ar the seeds can be impregnated with the condensed phosphate before introduction of the seeds into the fluidisedtbed. In general, it is preferred to impregnate the seeds in the very body of the fluidised bed, not only with the aqueous phase or phases but also with the condensed phosphate. In one method, the aqueous phases comprise an aqueous solution of hydrogen peroxide and an aqueous solution of sodium carbonate, which are introduced into the fluidised bed dryer separately using separate atomisers simultaneously and fin a second method, both solutions are introduced simultaneously through the same atomiser. In both methods, however, the fluidised bed dryer acts as a reactor in which formation of sodium percarbonate takes place by reaction between the sodium carbonate and the hydrogen peroxide. Alternatively, the aqueous phase can comprise a solution of sodium percarbonate, or possibly, a suspension, prepared prior to introduction of the phase into the fluidised bed, for example, by reaction between a solution of hydrogen peroxide and a solution of sodium carbonate, or by the action ob sodium peroxide on sodium bicarbonate in the presence of an aqueous solution of hydrogen peroxide. However, a particularly interesting method is to use a solution of hydrogen peroxide and a solution of sodium percarbonate, introduced simultaneously into the fluidised bed dryer separately or together. The preferred method is to introduce them together by the same atomiser.

When the seeds are impregnated outside the fluidised bed dryer, the seeds and the aqueous phase containing sodium percarbona,te in solu tion or possibly in suspension or comprising solutions of precursors for the sodium per carbonate are introduced into a mixer. Gener ally, the process employs a solution of sodium carbonate and a solution of hydrogen peroxide, which are fed into the mixer simultaneously.

However, when the aqueous phase contains sodium percarbonate, it can Ibe prepared by the methods described hereinbefore.

In practice, 'the phosphate is generally dissolved in at least one of the aqueous phases which would be used to impregnate the seeds, in processes using no condensed phosphate.

Thus, the condensed phosphate is usually dissolved in either or both of the hydrogen peroxide solution or the sodium carbonate solution, preferably the latter.

The amount of phosphate that can be used falls within fairly wide limits, preferably at least 0.01 g, and desirably up to 50 g of condensed phosphate per kg of sodium percarbonate, calculated as Na2CO,.3/2H2O2. Particularly preferably, the amount is between 0.1 and 20 g condensed phosphate per kg of sodium percarbonate on the same basis.

The concentration of hydrogen peroxide in its aqueous solution can fall within wide limits.

Advantageously, the aqueous solution contains from S to 70% by weight hydrogen peroxide and preferably from 15 to 40% by weight.

Lower concentrations of hydrogen peroxide, although feasible, are of little interest com mercially because the amount of water to be evaporated becomes very large. On the other hand, it is not advisable Ito employ higher con centratiions of hydrogen peroxide, because it can become dangerous to handle them.

The concentration of sodium carbonate in its aqueous solution can fall within fairly wide limits, the maximum being the solubility of sodium carbonate at the prevailing temperature of the solution. Desirably, the solution contains at least 5% Iby weight sodium carbonate and preferably from 10 to 35% by weight sodium carbonate, calculated as Na2CO8.

The mole ratio of hydrogen peroxide to sodium carbonate employed, either to form an aqueous phase containing sodium percarbonate, and form sodium percarbonate in sits, is generally about 1.5. Normally the mole ratio is between 1.3 and 1.7 and preferably between 1.45 and 1.52. Other additives can be introduced into either the hydrogen peroxide solution or the sodium carbonate solution, or both.

Such additives include stabilisers of sodium percarbonate or their precursors such as magnesium sulphate and sodium silicate, as well as corrosion inhibitors such as nitrates. Thus, in some embodiments, the aqueous hydrogen peroxide solution contains magnesium sulphate and the aqueous sodium carbonate solution contains sodium silicate. The staibllisers for sodium percarbonate often comprise from 0.1 to 20 g of stabiliser per kg of sodium percarbonate produot, calculated as Na2CO,.3/2H2O2, but their use is not essential.

The temperature of the fluidised bed is selected so as to avoid decornposition of sodium percarbonate, and is generally selected within the range Of ambient to 100"it, frequently between 35 and 95"C and preferably between 45" and 85 C. The temperature of the influent caraier gas, whether air or otherwise, intro diced at the bottom of ,the fluidised bed, e.g.

through a gauze or distribution plate, can fall within wide limits, and is dependent in par titular upon the desired bed temperature, the amount of water to be eliminated and the flow rate of the carrier gas. In general the influent temperature is within the range of 800 to 250"C and preferably between 1000 and 200 > C. However, it will be recognized that temperatures outside these ranges may be suit- able, depending upon the other parameters, but that higher temperatures are generally avoided, if possible, because they can cause losses of hydrogen peroxide by evaporation or decomposition.

The carrier gas can be any gas that is inent chemically relative to the cotiteents of the fiuidsed bed. Such Igases include nitrogen, the noble gases, oxygen or air, of which air is par ticolarly suitable. In general, the parameters, namely flow rate of the carrier gas, tempera trrreofthebed,iinconjunction with the rate of addition of aqueous phase directly or indirectly to the bed are controlled so as to maintain the humidity of the carrier gas at below 100%, and preferably at below 90%, so as to avoid or minimiise decomposition of the sodium percarbonate.

The seeds employed in the process of the present invention are selected to be smaller than 'the product granules. They can be pó- duced in various ways. At the start-up of a plant employing the process according to the present invention, the seeds are most preferably small particles of sodium percarbonate obtained either from another and similar plant or from the soiled wet process. Preferably, particles having appropriate dimensions but of other inorganic persalts, such as sodium perborate monohydrate or tetrahydrate, pefsilicates, persuphates or perhydrated phosphates can also be used, advantageously because they too ensure relatively uniform distribution of active oxygen in the product granules.

During operation of the fluidised bed, it twill be recognised that when impregnation takes place in the fluldised bed, at least a portion of the seeds comprise sodium percarbonate fines which are produced under normal conditions of operation. The amount of seeds can be increased by deLiberately introducing sodium percarbonate fines 'into the bed or by mechanically destroying some of the granules already formed within the bed itself or by using both methods simultaneously.

Sodium percarbonate fines which are fed into the bed can originate from reject product, after grinding outside the fluidized bed, or from excessively large granules of sodium per carbonate produced in the dryer or from the recycling of fines which were entrained in the effluent fluidising gas and carried out of the bed, or from manufacture in another process or from the pulverisation of sodium percarbonate particles or from several of these possible pro cesses simultaneously.

The 'fluidised bed dryer can be equipped with one or more devices which mechanically destroy agglomerates and at the same time form seeds, such as pulverisers, agitators, or scrapers. Use of such devices serve to prevent agglomeration and packing in the bed. In large fluidised beds, such devices generally are not employed because agglomeration is usually not a signiflcant problem, so that it is generally preferred to feed recycled sodium percabonate or waste 'into such fluidised beds, when it is desired toincrease the proportion of seeds in 'the bed. The preferred method of increasing the proportion of seeds in the bed comprises recycling the fines that have been entrained in the effluent fluidising gas and carried out of the bed.

When the seeds are impregnated outside the fluidised bed dryer, i.e. in a separate impreg nation stage, the fluidised bed is normally charged solely with the product from the im- pregnation stage, in which case the seeds are added to the mixer employed in the impregnation stage. The seeds will normally comprise recycled sodium percarbonate fines entrained in the fluidising gas. Alternatively, the sodium percarbonate fines can be obtained by pulverising over-sized sodium percarbonate granules, that ris to say particles having undesirably large dimensions sieved out of the fraotion of material withdrawn from the dryer that is taken as the product. The fraction of material that is taken as the product is relatively small, because it has been established in practice that the quantity of dry sodium percarbonate granules recycled to the mixer in which they are impregnated with, further aqueous phase is preferably such that the product leaving the mixes has a moisture content from below 5 to 10% by weight, depending upon the type of mixer used, in order to avoid the formation undesirably of agglomerates.

When Ithe seeds are impregnated in the fluidised bed dryer, the solution or solutions can be introduced at various points within the bed, but preferably in the lower half. The solutions can be fed into the fluidised bed at the same time separately through two different atomisers or through a single atomiser with pre-mixing being accomplished inside the atomiser or at its inlet. However, any other known method for introducing solutions into the fluidised bed can also be used.

The temperature of each solution can fall within fairly wide lirnits. Preferably, it does not exceed the prevailing temperature in the firiidised bed so as to avoid premature crystal isation in the atotnisers when concentrated solutions are employed. In general, the temperatures are selected within the range of from ambient ,to 70or, preferably between 20 and 50 C. Where two solutions are employed, they need not necessarily have the same temperature.

When the seeds are impregnated outside the fluidised bed, various types of mixers can be employed. Continuous mixers are very suitable, for example a screw mixer, or a rotating drum fitted with a sccaper, or a trough equipped with one or two shafts carrying paddles.

The process according to the present invention can be conducted as a continuous process or a batch process.

The fluldised bed dryer can conveniently be cylindrical, cylindroconical or parallelpipedic, but it will be recognised that other geotetrical shapes could also be used satisfactorily. The fluidised bed dryer can be charged with solid material using any method known in itself for this purpO, for example a Venturi system. In practice, .the charge usually has a mean particle diameter of below 0.4 term and mostly between 0.01 and 0.35 mm, but it will be apparent that such values are given only by way of example, and depending upon the desired product, the solid charge could have a diffe=Dt we size range. Discharge of the granules from the fluidised bed can be effected by any known device, for example by elutria tion through the bottom of 'the dryer or through a side tube fitted at the bottom of the dryer, or by over-flow through a side tube, this tube then determining the height of the fluidised bed.

Effluent gases from the fluidised bed pass through a fines separator, for example a cyclone. After the fines have been removed from them, rhe gases can be vented to atinos- phere or after their water vapour content has been dirniiished, preferably to zero, by drying or condensation, the gases can be partially or totally recycled.

When the impregnation of the seeds is accoplished directly in the fluidised bed dryer, the size of the granules depends largely on the quantity of seeds present in the bed, the coarseness of the granules being inversely proportional to the content of seeds. The coarseness of the granules will thus be inversely proportional to the quantity of fine graunles of sodium percarbonate introduced deliberately into the bed and to the extent of use of devices for miechanical break-up of the grains (pulver isers, etc.) if any are present in the bed. The size of the granules also depends on the pres sure of the gas in the jet or jets for feeding the solution into the fluidised bed, the coarseness of the granules being inversely proportional to the pressure. Adjustment of granule size to the required value can therefore easily be accompolished by varying the proportion of seeds in the bed, either by the introduction of seeds or by the internal 'break-up of the granules or by ,varying the pressure in the jets or by the simultaneous use of these two methods.

In a further aspect of the present invention there are provided sodium percarbonate gran ules containing from 0.001 to 5% by weight condensed phosphate as hereinbefore defined.

Preferably the granules contain from 0.01 to 2% by weight of the condensed phosphate.

Although any of the condensed phosphates described herein can be employed, the condensed phosphate is preferably sodium hexa met aphosph ate.

Sodium percarbonate granules containing condensed phosphate are suitable for all the customary uses of granular sodium percarbonate, and are particularly suitable as a con stituent of washing powders. It will be recog nised that in comparison with granules not containing condensed phosphate, the products according to the present invention demonstrate improved stability to accidental contamination with metal ions, such as iron, and in the pres ence of conventional constituents of washing powders. Moreover, the sodium percarbonate granules have retained their homogeneity.

Advantageously, in some embodiments, the process according to the present invention en ables reagents to be introduced by a single atomiser directly intro the fluidised bed without blockage of the atomiser occurring to the same extent as before. In addition, in some embodi ments a considerable improvement in the utiLisation of hydrogen peroxide is observed.

Plants which can be used to carry out the process of the present invention are described hereinafter with reference to and as illustrated by the accompanying drawings, in which: Figures 1, 2 and 3 show respectively cross sectional views in the vertical plane through three different plants.

The plant in Figure 1, comprises a vertical cylindrical fluidised bed equipped wilth means for charging the bed chamber with solid par tides, means for fluidising the solid particles m the chamber, means for introducing aqueous solutions into the bed and means for recover ing solid particles from effluent fluidising gas.

The cylindrical fluidised bed comprises a lower chamber or wind box 5, provided with a fluidising gas inlet comprising an air plipe 23 from a pump (not illustrated) leading to a preheater 3 having a regulator 4 and an air inlet line 2 leading to the wind box 5. The wind box 5 is divided from bed chamber 1 by a perforated plate 6 having a central orifice 15 through which the shaft of a fixed blade scraper 16 passes. The bed chamber 1 is pro vided with a Venturi tube 13, which is fed with solid particles from a vibrating conveyor 12, and air through air pipe 14, with a tube 24 through which a grinder can be introduced into the bed chamber 1 adjacent to the fixed blade scraper 16, with an overflow tube 22, and with a tube 27 through which solid particles can be elutriated. The bed chamber 1 also contains an atomiser 19 having an air feed line 20 coming from a preheated 21, a hydrogen peroxide feed line ;b8 coming from a thermostatic storage tank 17 and a sodium carbonate solution feed line 25 coining from a thermostatic storage tank 26. Above the bed chamber 1 is an expansion chamber 7, having an effluent fluidising gas outline line 8 feeding into a cyclone 9 having an air discharge ventilator line 10, and a hopper 111 for solid particles separated from the effluent fluidising gas.

In operation, the bed chamber 1 is charged with seed particles from hopper 111, conveyed along vibrating conveyor 12, into Venturi tube 13, and the particles are fluidised by gas pumped through gas line 2 into the wind box 5 and thence through the perforations in plate 6. Aqueous hydrogen peroxide is fed to the atomiser 19 through feed line 18, at the tem perature maintained in the thermostatic storage jtank 17, and aqueous sodium carbonate solution is fed through feed line 25 at the tempera- ture maintained in the thermostatic storage tank 26 and air through feed line 20, and the mixed solutions thereby sprayed onto the par tides in the bed chamber 1. The fixed blade scraper 16 rotates over the surface of the per forated plate 6, thereby preventing the particles sticking to the perforated plate 6 and blocking its perforations.

The plant shown in Figure 2, and ks method of operation, is identical to that shown in Fig ure 1, except that the bed chamber 1 is equipped with two atomisers, 19a and 19b, of which atomiser 19a is fed by aqueous hydrogen peroxide feed line 18 and air line 20a and atomiser 19b iis fed by aqueous sodium carbonate feed line 25 and air feed line 2Ob.

The plant shown in Figure 3 comprises separate pieces of apparatus for impregnating seeds with aqueous solution, for drying the impregnated seeds and for recycling part of the off-take from the dryer and for producing more seeds, more specifically, the plant comprises a screw mixer 28, which is fed by a vibrating trough 29 and a second vibrating trough 30 which is flitted with a feed hopper 31. The mixer 28 has an overflow outlet 34 feeding a vibrating sieve 35 which discharges into a fluidised bed 36. The bed 36 is equipped with a steam jacket 37, a base plate 38, a fluidising gas inlet line 53 and a fluidising gas outlet line 32, a partition 39 which divides bed 36 into two compartments and a dried product take-off tube 40. The gas outlet 1ine 32 leads via a cyclone for separating out solid particles and discharging them into hopper 31, to extr a lower or main chamber having a diameter of 152 mm and extending for 915 mm above the base plate, and the upper expansion chamber had a maximum diameter of 305 mm and a height of 300 mm The bed was charged initially with 3 kg of seeds of sodium percarbonate fines, and then fluidised with a current of air having an initial temperature of 1800C and a flow rate of 50 m1/h (normalised), producing a bed tern- perature of between 680 and 70"C. An aqueous solution containing 24.6% by weight hydrogen peroxide and 0.73% by weight magnesium sulphate heptahydrate, and having a temperature of 25 C was sprayed through an atomiser reaching into the very body of the fluidised bed at a flow rate of 1.494 kg/h. A second aqueous solution containing 30% by weight sodium carbonate, 1.2% by weight sodium silicate (36 Baum), sufficient ferric chloride for the product to have the iron content speci dried in Table 1, and 0.95% by weight sodium hexametaphosphate having the general formula Na^( PO3)6.1OH2O (hereinafter called HMP) except in comparative product 7R which contained no HMP, and having a temperature of 4S"C was sprayed into the fluidised bed through a separate atomiser at a flow rate of 2.58 kg/h. The mole ratio of hydrogen peroxide to sodium carbonate was 1.48:1. The atomisers were supplied with air having a temperature of 550 to 60 C at an effective piess- ure of 1 kg/rm2 and at a flow of 3 m3/h (normalised).

The stability of products 1 to 6, according to the present invention, and comparative product 7R was then measured by determining the loss of active oxygen during storage for 2 hours at 105 C. The test comprised holing a flat bottomed glass tube having a diameter of 28 5 mm and a height of 160 mm with 50 g of sodium percarbonate of known active oxygen content, placing the tube in a chamber thermo- statically controlled at 105 C, and after 2 hours removing the 'tube from the chamber, cooling it and pouring rites contents into a flask of 140 ml of 9 N sulphuric acid and titrating with permanganate to determine the residual active oxygen content.

The loss of active oxygen shown in Table 1 for each product is expressed as a percntage and given by the expression Co - Cf X 100 Co where C, and Cf are respectively the concen Oration of active oxygen of the product before and after the test.

The composition of the products and the results of the stability test are given in Table 1 below, products 1 to 6 being according to the present invention, and product 7R being present by way of comparison only. TABLE 1

Product No. 1 2 3 4 5 6 7R Content of iron mg kg 12 10 22 42 70 92 37 Content of active oxygen (C0) g/kg 144 138 144 144 143 144 137 Content of HMP g/kg 6.3 6.3 6.4 6.4 6.4 6.4 0 Mean particle diameter mm 0.360 0.425 0.370 0.400 0.355 0.340 0.425 Stabi li ty: loss of active oxygen % 7 7 7 8 8 9 11 From Table 1, it will be seen that the presence of HMP improved the stability of sodium percarbonate.

EXAMPLE 2.

This Example demonstrates the operational advantage of adding HMP bo one of the solu- tions which are sprayed into the fluidised bed using only one atomiser.

In this Example, the plant used was d the type shown in Figure 1, and described hereinbefore with respect thereto. The plant was identical to that employed in Example 1, with the exception that both the hydrogen peroxide and the sodium carbonate solutions were sprayed simultaneously into the fluidised bed through a single atomiser. The process conditions were identical to those described in Example 1, as was the hydrogen peroxide solution. The sodium carbonate solution employed in test 8 contained 30% by weight sodium carbonate, 1.2% by weight sodium silicate (36 Baumé) and 0.95% by weight HMP, and in comparison test 9R the HMP was omitted.

It was observed that test 8 could be con tinted for several days without the atomiser clogging, whereas in test 9R, the atomiser clogged rapidly, after about 5 minutes opera- tion. From comparison of tests 8 and 9R it will be seen that the presence of HMP makes it much more convenient to use a single atomiser, thus enabling a relatively simple plant to be employed.

EXAMPLE 3.

This Example demonstrates that use of a single atomiser together with incorporation of HMP in solution can result in better utilisation of the hydrogen peroxide.

The plant employed in this Example was identical to that used in either Example 1 or Example 2 depending upon whether two or one atomisers are used. In test 1OR, carried out for purposes of companson, two atomisers were used, and the sodium carbonate solution did not contain HMP. The active oxygen content of the sodium percarbonate product represented between 95.4% and 96.8% of the hydrogen peroxide introduced into the bed. In test 11, the process according to the present invention, a hydrogen peroxide solution and a sodium carbonate solution which were identical to those employed in test 10R, except for the inclusion of 6.6 g/kg HMP in the sodium carbonate solution were sprayed into the fluid bed through a sig single atomliser. The active oxygen content of the product represented between 94.4% and 98.1% of the hydrogen per oxide introduced into the fluidised bed.

It will be recognised, therefore, that the presence of HMP in the sodium carbonate solution enabled better utilisation to be made of the hydrogen peroxide.

WHAT WE CLAIM IS: 1. A process for the production of sodium percarbonate granules comprising the steps of:- 1. impregnating seeds as defined herein with an aqueous phase or phases containing sodium percarbonate or its precursors, 2. evaporating water from the impregnated seeds in a fluidised bed dryer in the presence of at least one condensed phosphate as herein defined.

2. A process as claimed in claim 1 wherein the condensed phosphate is a metaephosphate.

3. A process as claimed in claim 2 wherein the metaphosphate is hexametaphosphate.

4. A process as claimed in any of the preceding claims wherein the condensed phosphate is a sodium salt.

5. A process as claimed in any preceding claim wherein sufficient condensed phosphate .is introduced into withe flubidised bed to result in the granules of sodium percarbonate product containing from 0.01 to 50 g condensed phosphate per kg product.

6 A process as claimed in claim 5 wherein sufficient condensed phosphate is introduced into the fluidised bed to result in the granules of the sodium percarbonate product containing from 0.1 to 20 g condensed phosphate per kg product.

7. A process as claimed in any preceding claim wherein the seeds comprise small particles of sodium percarbonate.

8. A process as claimed in claim 7 wherein at least some of the seeds are particles which have been carried out of the fluidised bed in the effluent fluidising gas, and separated therefrom.

9. IA process as clained in any preceding claim wherein the seeds are impregnated with an aqueous solution d hydrogen ,peroxide and an aqueous solution of sodium carbonate, optionally mixed prior to impregnation.

10. A process as claimed in claim 9 wherein the condensed phosphate is dissolved in the hydrogen peroxide and/or sodium carbonate solution.

811. A process as claimed in claim 10 wherein the condensed phosphate is dissolved in the sodium carbonate solution.

12. A process as claimed in any preceding claim wherein the seeds are impregnated in the fluidised bed, so that steps 1 and 2 and both occurring lin the fluidised 'bed at ,the same time.

13. A process as claimed in claim 12 and any of claims 9 to 11 wherein the hydrogen peroxide solution and sodium carbonate solu tion are introduced into the fluidised bed through the same atomiser 14. A process as claimed in any preceding claim wherein the fluidised bed is maintained at a temperature of from 350 to 950C.

15. A process for the production of sodium percarbonate granules as claimed in claim 1 and employing plant substantially as shown in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. From Table 1, it will be seen that the presence of HMP improved the stability of sodium percarbonate. EXAMPLE 2. This Example demonstrates the operational advantage of adding HMP bo one of the solu- tions which are sprayed into the fluidised bed using only one atomiser. In this Example, the plant used was d the type shown in Figure 1, and described hereinbefore with respect thereto. The plant was identical to that employed in Example 1, with the exception that both the hydrogen peroxide and the sodium carbonate solutions were sprayed simultaneously into the fluidised bed through a single atomiser. The process conditions were identical to those described in Example 1, as was the hydrogen peroxide solution. The sodium carbonate solution employed in test 8 contained 30% by weight sodium carbonate, 1.2% by weight sodium silicate (36 Baumé) and 0.95% by weight HMP, and in comparison test 9R the HMP was omitted. It was observed that test 8 could be con tinted for several days without the atomiser clogging, whereas in test 9R, the atomiser clogged rapidly, after about 5 minutes opera- tion. From comparison of tests 8 and 9R it will be seen that the presence of HMP makes it much more convenient to use a single atomiser, thus enabling a relatively simple plant to be employed. EXAMPLE 3. This Example demonstrates that use of a single atomiser together with incorporation of HMP in solution can result in better utilisation of the hydrogen peroxide. The plant employed in this Example was identical to that used in either Example 1 or Example 2 depending upon whether two or one atomisers are used. In test 1OR, carried out for purposes of companson, two atomisers were used, and the sodium carbonate solution did not contain HMP. The active oxygen content of the sodium percarbonate product represented between 95.4% and 96.8% of the hydrogen peroxide introduced into the bed. In test 11, the process according to the present invention, a hydrogen peroxide solution and a sodium carbonate solution which were identical to those employed in test 10R, except for the inclusion of 6.6 g/kg HMP in the sodium carbonate solution were sprayed into the fluid bed through a sig single atomliser. The active oxygen content of the product represented between 94.4% and 98.1% of the hydrogen per oxide introduced into the fluidised bed. It will be recognised, therefore, that the presence of HMP in the sodium carbonate solution enabled better utilisation to be made of the hydrogen peroxide. WHAT WE CLAIM IS: 1. A process for the production of sodium percarbonate granules comprising the steps of:-

1. impregnating seeds as defined herein with an aqueous phase or phases containing sodium percarbonate or its precursors,

2. evaporating water from the impregnated seeds in a fluidised bed dryer in the presence of at least one condensed phosphate as herein defined.

13. A process as claimed in claim 12 and any of claims 9 to 11 wherein the hydrogen peroxide solution and sodium carbonate solu tion are introduced into the fluidised bed through the same atomiser

14. A process as claimed in any preceding claim wherein the fluidised bed is maintained at a temperature of from 350 to 950C.

any one of Figures 1, 2 and 3 and described herein with respect thereto.

16. A process for the production of sodium percarbonate granules substantially as described herein test 8 in Example 2 or test 11 in Example 3.

17. Sodium percarbonate granules whenever produced by a process as claimed in any pre-, ceding claim.

18. Sodium percarbonate granules as claimed in claim 17 contaiining from Q001 to 5% by weight condensed phosphate.

19. Granules as claimed in claim 18 containing from 0.01 to 2% by wight condensed phosphate.

20. Granules as claimed in claim 18 or 19 wherein the phosphate is a metaphosphate.

21. Granules as claimed in claim 20 wherein the metaphosphate is hexametaphosphate.

22. Granules as claimed in any of claims 18 to 21 wherein the sodium salt of the condensed phosphate is used.

23. Sodium percarbonate granules containing condensed phosphate and substantially as described herein with respect to any of products 1 to 6 in Example 1, or the product of test 8 in Example 2 or test 11 in Example 3.

24. A washing composition containing sodium percarbonate granules as claimed in any of claims 17 to 22.